Etymology tree
Proto-Indo-European *h₂éd
Proto-Italic *ad
Medieval Latin ad
Proto-Indo-European *dyew-
Proto-Indo-European *-ós
Proto-Indo-European *deywós
Proto-Italic *deiwos
Old Latin deivosder.
Latin deus
Medieval Latin Deum
Medieval Latin ad Deumder.
Old French a Dieu vos comant
French adieuubor.
German Adieu
Unadapted borrowing from French adieu.