1580s in figurative sense, c. 1600 in literal sense (transitive and intransitive), from un- (2) "reverse, opposite of" + mask (v.). Related: Unmasked; unmasking.
Entries linking to unmask
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)).
mask v.
1560s, "to take part in a masquerade" (a sense now obsolete); 1580s as "to wear a mask," also "disguise (feelings, etc.) under an assumed outward show;" from mask (n.) and French masquer. Military sense of "conceal" (a battery, etc.) from the view of the enemy" is from 1706. Related: Masked; masking. Masking tape recorded from 1927; so called because it is used to block out certain surfaces before painting.