prefix of negation, Old English un-, from Proto-Germanic *un- (source also of Old Saxon, Old Frisian, Old High German, German un-, Gothic un-, Dutch on-), from PIE *n- (source of Sanskrit a-, an- "not," Greek a-, an-, Old Irish an-, Latin in-), combining form of PIE root *ne- "not." Often euphemistic (such as untruth for "lie").
The most prolific of English prefixes, freely and widely used in Old English, where it forms more than 1,000 compounds. It underwent a mass extinction in early Middle English, but emerged with renewed vigor 16c. to form compounds with native and imported words. It disputes with Latin-derived cognate in- (1) the right to form the negation of certain words (indigestable/undigestable, etc.), and though both might be deployed in cooperation to indicate shades of meaning (unfamous/infamous), typically they are not.
It also makes words from phrases (such as uncalled-for, c. 1600; undreamed-of, 1630s; uncome-at-able, 1690s; unputdownable, 1947, of a book; un-in-one-breath-utterable, Ben Jonson; etc., but the habit is not restricted to un-; such as put-up-able-with, 1812). As a prefix in telegramese to replace not and save the cost of a word, it is attested by 1936.
c. 1300, "to strip (someone) of clothes, strip a slain enemy," from Old French espillier "to strip, plunder, pillage," from Latin spoliare "to strip, uncover, lay bare; strip of clothing, rob, plunder, pillage," from spolia, plural of spolium "arms taken from an enemy, booty;" originally "hide, skin stripped from a killed animal," from Proto-Italic *spolio- "skin, hide," from PIE *spol-yo-, probably from a root *spel- (1) "to split, to break off" (see spill (v.)) on the notion of "what is split off."
From late 14c. in English as "strip with violence, rob, pillage, plunder, dispossess; impoverish with excessive taxation." Used c. 1400 as the verb to describe Christ's harrowing of Hell. Sense of "destroy, ruin, damage so as to render useless" is from 1560s; that of "to over-indulge" (a child, etc.) is from 1640s (implied in spoiled). Intransitive sense of "become tainted, go bad, lose freshness" is from 1690s. To be spoiling for (a fight, etc.) is from 1865, from notion that one will "spoil" if he doesn't get it.