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)).
"close tightly (a hole), fill or stop with or as with a plug," 1620s, from plug (n.) or from Dutch pluggen. OED dates the meaning "work energetically at" from c. 1865, and cites "Remembered on the river at Oxford" (and one wonders if a memory from Cambridge would pass). Sense of "popularize by repetition" is from 1906. Slang sense "put a bullet into" is recorded from 1870. Related: Plugged; plugging.
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Definitions of unplug
pull the plug of (electrical appliances) and render inoperable;