When people say goodbye in German, they don’t usually say Auf Widersehen, they mostly say tschüß. I always used to wonder where this odd word comes from. Now I know. “In rapid speech, its possibly Wallonian variant adjuus (sounding close to Spanish adiós) yielded the almost unrecognizable word tschüs.” Monthly gleanings for October 2019 | OUPblog
I have always seen the word spelled with a double ss (or the more or less equivalent ß). But I see that Duden permits both spellings. Duden | tschüs | Rechtschreibung, Bedeutung, Definition, Herkunft
In case you’re wondering, the title of this post comes from a sickly scene in the Sound of Music: it is the refrain the von Trapp children sing to the party guests on their way up to bed.