Etymology
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Words related to au

ad- 

word-forming element expressing direction toward or in addition to, from Latin ad "to, toward" in space or time; "with regard to, in relation to," as a prefix, sometimes merely emphatic, from PIE root *ad- "to, near, at."

Simplified to a- before sc-, sp- and st-; modified to ac- before many consonants and then re-spelled af-, ag-, al-, etc., in conformity with the following consonant (as in affection, aggression). Also compare ap- (1).

In Old French, reduced to a- in all cases (an evolution already underway in Merovingian Latin), but written forms in French were refashioned after Latin in 14c. and English did likewise 15c. in words it had picked up from Old French. In many cases pronunciation followed the shift. Over-correction at the end of the Middle Ages in French and then English "restored" the -d- or a doubled consonant to some words that never had it (accursed, afford). The process went further in England than in France, where the vernacular sometimes resisted the pedantic, resulting in English adjourn, advance, address, advertisement (Modern French ajourner, avancer, adresser, avertissement). In modern word-formation sometimes ad- and ab- are regarded as opposites, but this was not in classical Latin.

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au revoir (interj.)
1690s, French, "good-bye for now," literally "to the seeing again." From au "to the" (see au) + revoir "see again, see in turn" (Old French reveoir, 12c.), from Latin revidere, from re- "back, again" (see re-) + videre "to see" (see vision).
a la 
from French à la, literally "to the," hence "in the manner of, according to," from à, from Latin ad "to" (see ad-) + la, fem. of definite article le "the," from Latin ille (fem. illa; see le). Attested in English in French terms from fashion or cookery since late 16c.; since c. 1800 used in native formations with English words or names.
aureate (adj.)
early 15c., "resembling gold, gold-colored," also figuratively, "splendid, brilliant," from Latin aureatus "decorated with gold," from aureus "golden," from aurum "gold," from PIE root *aus- (2) "gold" (source also of Sanskrit ayah "metal," Avestan ayo, Latin aes "brass," Old English ar "brass, copper, bronze," Gothic aiz "bronze," Old Lithuanian ausas "gold"), which is probably related to root *aus- (1) "to shine."

Especially of highly ornamented literary or rhetorical styles. Related: Aureation.
au courant (adj.)
"aware of current events," 1762, French, "with the current, in the current (of events);" see au + current (n.).
au fait (adj.)
1743, French, "to the point, to the matter under discussion," literally "to the fact," from au "to the" (see au) + fait "fact" (see feat). Used in French with sense of "acquainted with the facts, expert, fully skilled."
au naturel (adj.)

1817, "uncooked," French, literally "naturally, in the natural state." Originally in English a term in French recipes, it was used euphemistically in English for "undressed" by 1860, perhaps via its use in French in the visual arts. See au + natural (adj.).

au pair (n.)
1897 of the arrangement, 1960 of the girl; French, literally "on an equal footing" (see au + pair (n.)).