"Farewell" works for that purpose in English. And since you reminded me, that's the meaning of "sayonara" too -- you're not supposed to use it for casual goodbyes.

Same deal with Portuguese ⟨adeus⟩. With the role of casual valediction being taken by a borrowing, ⟨tchau⟩ [tʃäʊ̯] ~ ⟨xau⟩ [ʃäʊ̯]… etymologically way more problematic, given it's ultimately from Venetan ⟨sciao⟩ [stʃäʊ̯] "slave". Originally ⟨sciao vostro⟩ "your slave" (implied: "I'm at your service").

Glad etymology doesn't dictate current meaning though.