French, "at the, to the," from Old French al, contraction of a le, with -l- softened to -u-, as also poudre from pulverem, chaud from calidus, etc. Used in many expressions in cookery, etc., which have crossed the Channel since 18c., such as au contraire, literally "on the contrary;" au gratin, literally "with scrapings;" au jus, literally "with the juice."
