Adieu
late Middle English: from Old French, from a ‘to’ + Dieu ‘God’; compare with adios.
wiktionary
From Middle English adieu also adew, adewe, adue, from Old French adieu(“to God”), a shortening of a Dieu vous comant(“I commend you to God”), from Medieval Latin ad Deum(“to God”). Doublet of adios.
etymonline
adieu (interj.)
late 14c., adewe, from Old French a Dieu, a Deu, shortened from phrases such as a dieu (vous) commant "I commend (you) to God," from a "to" (see ad-) + dieu "God," from Latin deum, accusative of deus "god," from PIE *deiwos "god" (from root *dyeu- "to shine," in derivatives "sky, heaven, god"). Originally said to the party left ( farewell was to the party setting forth), but in English used as a general parting salutation. As a noun, "expression of kind wishes upon departure," late 14c. Compare native parting salutation good-bye, a contraction of God be with ye.
