
German word tschüss comes from Dutch and later Low German rendering of the French adieu (a dated form of "farewell" or "goodbye")
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adieu (French)
(Southern France) hello. (dated) goodbye, see you soon. Farewell, adieu Farewell.
adjuusjes (Dutch)
adjuus (Dutch)
adjüs (Low German)
(in many dialects) bye
tschüss (German)
(informal) bye, cheers, ciao.