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 fit a bow with a string," from string (n.). Meaning "to thread (beads, etc.) on a string" is from 1610s. Of musical instruments from 1520s (stringed instrument is from c. 1600). To string (someone) along is slang from 1902; string (v.) in the sense "deceive" is attested in British dialect from c. 1812; perhaps ultimately from the musical instrument sense and with a notion of "to 'tune' someone (for some purpose)." Related: Stringed (later strung); stringing.