"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."
Specific meaning "day before a saint's day or festival" is from late 13c. Transferred sense of "the moment right before any event, etc." is by 1780. Even (n.), evening keep the original form.
"having the characteristics of a plane," 1560s, from French plan, from Latin planus "flat, level, even" (see plane (n.1)).