mid-15c., "divest of armor," from un- (2) "opposite of" + harness (v.). Similar formation in Dutch ontharnassen "to disarm." Meaning "to free (a horse) from harness" is recorded from 1610s. Related: Unharnessed; unharnessing.
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)).
"to put a harness on a draught animal," c. 1300, from Old French harneschier "make ready, equip, arm," from harnois (see harness (n.)); figurative sense "to control for use as power" is from 1690s. Related: Harnessed; harnessing.