"concerning, about, in respect or reference to," c. 1200, onont "on level with, beside," also "in the company of, fronting against," a contraction of Old English on efn "near to, close by," literally "on even (ground with);" see a- (1) + even (adj.).
As an adverb, c. 1400, anents, anentes, with adverbial genitive. The unetymological -t was added 12c. Compare German neben "near to, by the side of," short for in eben, from Old High German ebani "equality."