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 fasten with (or as with) a seal," c. 1200, from seal (n.1). Meaning "to place a seal on (a document)" is recorded from mid-14c.; hence "to conclude, ratify, render official" (late 15c.). Sense of "to close up with wax, lead, cement, etc." is attested from 1660s, from the notion of wax seals on envelopes. In reference to the actions of wood-coatings, 1940. Related: Sealed; sealing. Sealing-wax is attested from c. 1300. To seal (one's) fate (1799) probably reflects the notion of a seal on an execution warrant.