Entries linking to evenness
Old English efen "level," also "equal, like; calm, harmonious; equally; quite, fully; namely," from Proto-Germanic *ebna- (source also of Old Saxon eban, Old Frisian even "level, plain, smooth," Dutch even, Old High German eban, German eben, Old Norse jafn, Danish jævn, Gothic ibns). The adverb is Old English efne "exactly, just, likewise." Modern adverbial sense (introducing an extreme case of something more generally implied) seems to have arisen 16c. from use of the word to emphasize identity ("Who, me?" "Even you").
Etymologists are uncertain whether the original sense was "level" or "alike." Used extensively in Old English compounds, with a sense of "fellow, co-" (as in efeneald "of the same age;" Middle English even-sucker "foster-brother"). Of numbers, from 1550s. Sense of "on an equal footing" is from 1630s. Rhyming reduplication phrase even steven is attested from 1866; even break (n.) first recorded 1907. Even-tempered from 1712. To get even with "retaliate upon" is attested by 1833.
word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).