Where did Welsh and Irish words come from?

In this post, I look at words inherited by both Welsh and Irish from:

• Proto-Indo-European (PIE), the presumed ancestor of the Indo-European family of languages (52 words).
• Proto-Celic, the presumed ancestor of the Celtic languages (8 words).

The information comes from the IE-COR database—a database of cognates for 170 words (lexemes) in 109 modern and 52 older Indo-European languages. IE-COR is available at IE-CoR – Home (clld.org).

I’ve written before about IE-COR at:

• Database of Indo-European cognates – Language Miscellany
• https://languagemiscellany.com/2026/03/where-did-russian-words-come-from-4/
• https://languagemiscellany.com/2025/11/albanian-words-of-unknown-or-uncertain-origin/

IE-COR covers lexemes for 170 meanings, including 175 lexemes in Welsh (covering all 170 meanings) and 171 lexemes in Irish (also 170 meanings). The Welsh lexemes are for North Welsh and the Irish lexemes are for modern standard Irish, as interpreted by a Galway speaker.

Words inherited from PIE

IE-COR identifies 52 words that both Welsh and Irish are believed to have inherited from Proto-Indo-European (22 nouns, 10 adjectives, 15 verbs and 5 numbers).

Nouns

Of the 22 nouns inherited by both Welsh and Irish from PIE, 12 are also found in modern languages of other Indo-European sub-families (table 1), and 10 are now found mainly or only in Celtic languages (table 2).

Table 1. Nouns inherited by Welsh and Irish from PIE, and still found outside Celtic

An asterisk *before a form shows that the form is reconstructed and is not attested by written records. A question mark ? before a word shows that there is some doubt about the form.

Some of the Welsh / Irish pairs are obviously related to each other (eg afon / abhainn), but it is much harder to see that some other pairs are related (eg dynes / bean).

There are some signs of what look like regular sound correspondences. For example, initial s- in PIE seems to be carried through into s- in Irish (in salann, síol) but changed to Welsh h- (in halen, hedyn).

Comments on table 1

1. Bird. Welsh (a)deryn and Irish éan. From PIE *pet- ‘to fly, fall’. Also in Italic (mainly Spanish pájaro, Romanian pasăre, Portuguese pássaro), Albanian (only Gheg shpend) and Armenian tṙčown. The PIE *pet- root is reconstructed as giving rise to words meaning ‘bird’, ‘fly’ (verb) and ‘wing’.
2. Egg. Welsh wy and Irish ubh. From PIE *h₂ōu̯ió- (‘egg’), also detectible in Italic (eg Portuguese ovo), Germanic (English egg), Slavonic (eg Slovene jajce), Greek (Ancient) ō̧ón; (Modern) aygó), Armenian (jow) and Iranic (eg Pashto agai)
3. Flower. Welsh blodyn and Irish bláth. From PIE *bʰleh₃- ‘to bloom’. Also in Germanic (eg German Blume), Italic (eg Spanish / Portuguese / Catalan flor, Romanian floare—borrowed by English as flower).
4. Knee. Welsh pen-glin and Irish glúin. PIE *g̑enu- (‘knee’) also led to words in Romance (eg Catalan genoll), Germanic (eg English knee), Greek (Modern) gónato, Armenian, Indic (Pali jānu, Sinhalese daṇahisa), Iranic (eg Pashto zangoon), Nuristani, and the extinct branches Anatolian and Tocharian.
5. Nail. Welsh ewin and Irish ionga. From PIE *h₃neg⁽ʷ⁾ʰ- ‘(finger)nail’. Also led to words in Slavonic (eg Czech neget), Baltic (eg Latvian nogs), Germanic (eg German Nagel), Italic (eg French ongle), Hellenic (eg modern Greek νύχι, nýchi), Armenian (elowng), Indic (eg Bengali nakh), -Iranic (eg Persian nāxun), Anatolian and Tocharian.
6. Name. Welsh enw and Irish ainm. PIE *Hneh₃-mn̥ (‘name’) has descendants in 152 languages in 13 branches: Slavonic (eg Russian imâ, Old Church Slavonic imę, Polish imię), Baltic, Germanic, Italic, Celtic, Hellenic, Albanian (emër), Armenian, Indic, Iranic, Nuristani, Anatolian and Tocharian.
7. River. Welsh afon and Irish abhainn. from PIE *h₂ebʰ. Root perhaps cognate with PIE *h₂ep- (‘water, river’, source of Iranic forms (eg Persian āb) and some Indic forms (eg Pali āpa, Kashmiri aab) and/or PIE *h₂ep– (source of Latvian upe, Lithuanian ùpė and words in Nuristani).
8. Root. Welsh gwreiddyn and Irish fréamh. From PIE ?*u̯reh₂d-, which has descendants in Germanic (eg German Wurzel), Italic (eg Spanish / Portuguese raiz, Italian radice, Romanian rădăcină), Hellenic (eg Ancient Greek rhíza, modern Greek ríza) and Albanian (rrënjë).
9. Salt. Welsh halen and Irish salann. PIE *sal- ‘salt’ led to lexemes in 95 languages in 8 branches: Slavonic (eg Russian sol’), Baltic (mainly Latvian sāls), Germanic (Danish, Norwegian, Swedish, Icelandic, English salt, Dutch zout, German Salz), Italic (eg Italian sale, Spanish Portuguese / Catalan sal, Romanian sare), Celtic, Hellenic (eg Ancient and New Testament Greek háls, Modern Greek aláti), Armenian (ał) and Tocharian.
10. Seed. Welsh hedyn and Irish síol. The PIE *seh₁- ‘to press in’, later ‘to sow’ comes up in 68 languages in 5 branches: Slavonic (eg Russian semâ, Czech / Slovak semeno), Baltic (eg Latvian sēkla), Germanic (eg German Samen) and Italic (eg Latin sēmen Spanish semilla) and Celtic.
11. Tongue. Welsh tafod and Irish teanga. Lexemes derived from PIE *dn̥g̑ʰu̯éh₂- ‘tongue’ exist in 134 languages across 10 branches: Slavonic (eg Russian âzyk, Polish język), Baltic (Lithuanian liežùvis), Germanic (eg German Zunge), Italic (eg Italian / Portuguese lingua, Romanian limbă), Celtic, Armenian (lezow), Indic (eg Hindi jībha), Iranic (eg Ossetic ævzag), Nuristani and Tocharian.
12. Woman. Welsh dynes and Irish bean. Lexemes from PIE *gʷen- ‘woman’ are to be found in 78 languages in 10 branches: Slavonic (Russian ženŝina, most others žena, žona or similar), Baltic (only extinct Old Prussian genno), Germanic (mainly Danish kvinde, Norwegian kvinne, Swedish kvinna, Icelandic kona), Celtic, Hellenic (eg Ancient / New Testament Greek gynḗ, Modern Greek gynaíka), Armenian (kin), Indic (only Sinhalese gǣni, Kashmiri zayn), Iranic (eg Persian zan), Anatolian and Tocharian.

Table 2. Nouns inherited by Welsh and Irish from PIE, and now found mainly or only in Celtic

1. Ash (burnt remains). Welsh lludw and Irish luaith. From PIE *leh₃u̯-. Only in Celtic.
2. Bark (of tree). Welsh rhisgl and Irish rúsc. From PIE *reu̯H- Only in Celtic. borrowing or loan-adaptation between Brythonic and Goidelic and/or early Romance may still be possible
3. Belly. Welsh bol and Irish bolg. From PIE bʰelg̑ʰ- ‘swell’. In some, but not all Celtic, and nowhere else.
4. Ear. Welsh clust and Irish cluas. From PIE *k̑leu̯s-  (‘listen to’). Only in Celtic.
5. Fire. Welsh tân and Irish tine. From PIE *tep- ‘to be warm, hot’. Only in Celtic.
6. Hand. Welsh llaw and Irish lámh. From PIE *pleh₂-. Only in Celtic.
7. Leaf. Welsh deilan and Irish duilleog. From PIE *dʰalh₁- ‘to spring up, sprout’. Not found outside Celtic, though there may be a link to Latin folium (‘leaf’)—and its Romance descendants.
8. Leg. Welsh coes and Irish cos. Not found outside Welsh, Irish and Scottish Gaelic, except Italian (only Neapolitain coscia)
9. Snake. Welsh neidr and Irish nathair. From PIE *(s)neh₁-  Now found with that meaning only in Celtic. Descendants of this PIE root did survive into older forms of Germanic (Old English næddre, Old High German nātara, Old Saxon nâdra), but in the modern languages came to mean specific types of snake: German Natter (‘adder, viper’), English Adder. (English re-analysed a nadder several centuries ago as an adder.)
10. Wing. Welsh aden and Irish eiteog. From PIE *pet- ‘to fly, to fall’, which also supplied words for ‘bird’ and ‘fly’ (verb). With the meaning ‘wing’, this root also has descendants in Germanic (only Old English fiþere, Old High German feddāh), Italic (mainly Latin penna), Hellenic (Ancient and NT Greek πτέρυξ ptéryx, modern Greek φτερούγα fteroýga), Armenian t’ew, Anatolian (Hittite pattar / pattan-) and Nuristani.

Adjectives

Table 3 lists 10 adjectives that both Welsh and Irish are believed to have inherited from Proto-Indo-European.

Table 3. Adjectives inherited by Welsh and Irish from PIE

Of these 10 adjectives, the first 3 are found in many other Indo-European languages, but the other 7 are found in few or none of the modern languages outside Celtic.

1. Full. Welsh llawn and Irish lán. PIE *pleh₁- (‘full’) has descendants in 127 languages across Slavonic (eg Russian polnyj), Baltic, Romance (eg French plein), Germanic (eg English full), Celtic (eg Irish lán), Greek (ancient) plḗrēs, Albanian (plot), Armenian, Indic (eg Hindi / Urdu pūrā), Iranic (eg Persian—Teheran por) and Nuristani.
2. New. Welsh newydd and Irish nua. Lexemes derived from PIE *neu̯- ‘new’ and meaning ‘new’ are found in 139 languages across Slavonic, Baltic, Germanic, Italic, Celtic, Hellenic, Armenian, Indic, Iranic, Anatolian and Tocharian.
3. Right. Welsh dde and Irish deas. From PIE *dek̑s-, which features in Romance (eg Latin dexter, Italian destro), Germanic (Old High German zesauua), some Slavonic (eg Serbo-Croat desni) Baltic (Lithuanian dešinė̃), Celtic, Greek (Ancient and Modern) dexiós, Indic (eg Benglai ḍān), Iranic and Nuristani.
4. Big. Welsh mawr and Irish mór. From meh₁-. The same adjective is attested a compound member in Slavonic Volodi-měrŭ [presumably source of the names Vladimir (Russian) and Voldymir (Ukrainian)]. Only in Celtic.
5. Cold. Welsh oer and Irish fuar. Derived from PIE *h₃eu̯g̑- ‘cold’.   Outside Celtic, found only in Baltic (Latvian auksts)
6. Old. Welsh hen and Irish nua. From PIE *sen-. Outside Celtic, found only in Armenian hin, Indic (only Early Vedic sána-), Baltic (only Lithuanian sẽnas), and Celtic lexemes derived from PIE *sen- ‘old’ 
7. Sweet. Welsh melys and Irish milis from PIE *mel(it)- ‘honey’. Outside Celtic, found only in Albanian ëmbël and extinct Anatolian.
8. Wide. Welsh llydan and Irish leathan from PIE *pleth₂- ‘become wide, spread oneself out’. Also found in Hellenic (New Testament platys and some other) Armenian layn, Indic (only Early Vedic pr̥thú– and Sinhgalese paḷal) Iranic (eg Parthian / Middle Persian frāx), Baltic (eg Latvian plats).
9. Black. Welsh du and Irish dubh. From PIE dʰeu̯b. Only in Celtic. Traditionally (but dubiously) connected to Greek τυφλός ‘blind’
10. Heavy. Welsh trwm and Irish trom. Only in Celtic. Derived from PIE *treu̯d- ‘to thrust, push’.

Verbs

Of the 15 verbs that both Welsh and Irish are believed to have inherited from Proto-Indo-European, 4 are also found in many other modern Indo-European languages (table 4).

For another 4 verbs (also found in many of the modern languages), Welsh continues a PIE verb, but Irish uses a construction involving bí (‘be’) alongside a noun descending from the same PIE verb (table 5).

The other 7 verbs are found in few or no modern Indo-European languages outside Celtic (table 6).

Table 4. Verbs inherited from PIE, also found in many other branches

• Drink. Welsh yfad and Irish ól are from PIE *peh₃(i̯)-. Also the source of Slavonic (eg Russian пить /pit’/), Baltic (Old Prussian poūton), Italic (eg French boire, Italian bere, Spanish / Portuguese beber), Greek (Ancient and Modern pínō), Indic (eg Hindi / Urdu pīnā), Classical Armenian əmpem, Iranic and Nuristani.
• Grind.  Welsh malu and Irish meil from PIE *melh₂- ‘to crush, grind’ . Also source of Slavonic (eg Serbo-Croat mljeti), Baltic (eg Latvian malt), Germanic (eg German mahlen, Norwegian / Danish male), Italic (eg French moudre, Spanish moler)
• Sing.  Welsh canu and Irish can. Celtic and Italic lexemes derived from PIE *⁽k̑⁾an- ‘to sing, sound (eg French chanter, Spanish / Portuguese / Catalan cantar)
• Swim.  Welsh nofio and Irish snámh, from PIE *(s)neh₂- ‘to bathe, swim. Also found in Italic (eg Spanish / Portuguese nadar, Hellenic (only Ancient Greek νέω néō), Indic (only Gawri sāneṭä dǟg and Khowar usneek), Iranic (eg Persian šenā kardan) and Tocharian.

Table 5. Verbs inherited from PIE (but Irish uses a noun)

• Know. Welsh gw(y)bod and Irish bí a fhios ag. From the perfect stem *u̯oi̯d- ‘to know’ (< *’to have seen’) from PIE *u̯ei̯d- ‘to catch sight of’. Also found in Germanic (eg German Wissen, Swedish veta), Slavonic (eg Czech vědět), Hellenic (Ancient and NT Greek οἶδα / oĩda/), Armenian g(i)tenal, Indo-Iranic (only early Vedic vid-, younger Avestan vaēδa).
• Live. Welsh byw and Irish bí beo. PIE *gʷi̯eh₃- ‘to live’ occurs in 105 languages in 10 branches: Slavonic (eg Serbo-Croat živjeti, Polish żyć, Ukrainian žiti), Baltic (eg Lithuanian dzeivuot), Italic (eg Italian vivere), Celtic, Armenian, Hellenic (eg Modern Greek zō), Indic (eg Hindi / Urdu jīnā), Iranic (eg Pashto jwande, Persian zendegī kardan), Nuristani and Tocharian.
• Sit. Welsh ista and Irish bí ina shuí. From PIE *sed- ‘to sit (down), seat onself’. Also found in Baltic (eg Lithuanian sėdė́ti), Slavonic ( eg Polish siedzieć), Germanic (eg German sitzen, one modern Hellenic form (Italiot kathénnō), Italic (eg Spanish / Portuguese estar sentado), some Indic (eg Pali nisīdati),  Iranic (eg Persian nešastan), Armenian nstel, Tocharian.
• Stand. Welsh sefyll and Irish bí na sheasamh. Derivatives from PIE *steh₂- ‘to position (oneself) are in 101 languages over 9 branches: Slavonic (eg Russian stoât’, Czech stát), Baltic (eg Latvian stāvēt), Germanic (eg Danish / Norwegian / Swedish stå), Italic (eg Latin stāre), Hellenic (eg Ancient and New Testmanent Greek hístēmi), Indic (eg Pali hiẏ hûwā), Iranic (eg Persian īstādan), Celtic and Anatolian. For more on Russian stoât’, please see https://languagemiscellany.com/2026/01/where-did-russian-words-come-from/  

Table 6. Verbs inherited from PIE, no longer found much outside Celtic

• Breathe. Welsh anadlu and Irish análaigh from PIE *h₂enh₁- ‘to breathe’. Also in Germanic (mainly in Scandinavia, eg Swedish andas) and some languages no longer spoken (early Vedic, Khtanese, Old Occitan, Old Catalan), Tocharian.  
• Fly. Welsh hedfan / hedag and Irish eitil from PIE *pet- ‘to fly, fall’. Also in Hellenic (eg Modern Greek petṓ), Armenian tṙčel and a few older languages. PIE *pet- also supplied words for ‘bird’ and ‘wing’. North Welsh also has fflio, borrowed from English fly.
• Hear.Welsh clwad and Irish clois from PIE *k̑leu̯- ‘to hear’. Also source of Indic (eg Nepali sunnu), Armenian lsel.
• Count.  Welsh cyfri and Irish áirigh. Also found in Greek (Ancient and New Testament) ἀριθμέω /arit^hmo/ and extinct Gothic.
• Hunt. Welsh hela and Irish seilg  from Proto-Celtic *selgā ‘hunt’, a derivative of PIE *selg̑- ‘to let go, release, send away’. Only in Celtic
• Run. Welsh rhedeg and Irish rith from PIE *ret- ‘to run. Only in Celtic.
• Vomit. Welsh chw(y)du and Irish sceith, from PIE *sk̑ʰei̯-. Only in Celtic.

Numbers

The PIE sources for the Welsh and Irish:

• numerals 2-5 have descendants in at least 155 of the 170 languages in IE-COR.
• numeral ‘one’ has descendants in 136 of the languages. 

The English numerals 1-5 all also have the same sources as their Welsh and Irish counterparts.

Table 7. Number 1 to 5

Proto-Celtic

Table 8 lists 8 words that both Welsh and Irish are believed to have inherited from Proto-Celtic.

Table 8. Words from Proto-Celtic