unfit adj.

prefix of reversal, deprivation, or removal (as in unhand, undo, unbutton), Old English on-, un-, from Proto-Germanic *andi- (source also of Old Saxon ant-, Old Norse and-, Dutch ont-, Old High German ant-, German ent-, Gothic and- "against"), from PIE *anti "facing opposite, near, in front of, before, against" (from PIE root *ant- "front, forehead," with derivatives meaning "in front of, before").

More or less confused with un- (1) through similarity in the notions of "negation" and "reversal;" an adjective such as unlocked might represent "not locked" (un- (1)) or the past tense of unlock (un- (2)).

c. 1400, "to marshal or deploy (troops);" early 15c. as "be fitting or proper, be suitable," from fit (adj.) and perhaps in part from Scandinavian (compare Old Norse fitja "knit"). From 1580s as "be the right shape." Transitive sense of "provide with what is suitable" is from 1590s; that of "make fit or suitable, bring into corresponding form or condition" is from c. 1600. Related: Fitted; fitting. Fitted sheets is attested from 1948.