1580s, from un- (2) "opposite of" + chain (v.). Figurative sense of "to liberate" is recorded from 1793. Related: Unchained; unchaining.
Entries linking to unchain
un- 2
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)).
chain v.
late 14c., "to bar with a chain; to put (someone) in chains," also "to link things together," from chain (n.). Related: Chained; chaining.