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.
stop v.
Old English -stoppian (in forstoppian "to stop up, stifle"), a general West Germanic word, cognate with Old Saxon stuppon, West Frisian stopje, Middle Low German stoppen, Old High German stopfon, German stopfen "to plug, stop up," Old Low Frankish (be)stuppon "to stop (the ears)."
These words are said by many sources to be a Germanic borrowing of Vulgar Latin *stuppare "to stop or stuff with tow or oakum" (source of Italian stoppare, French étouper "to stop with tow"), from Latin stuppa "coarse part of flax, tow." In support of this theory, it is said that plugs made of tow were used from ancient times in Rhine valley. Century Dictionary says this "suits phonetically," but "is on grounds of meaning somewhat doubtful." Barnhart, for one, proposes the whole Germanic group might be native, from a base *stoppon.
Sense of "bring or come to a halt, discontinue" (mid-15c.) is from notion of preventing a flow by blocking a hole, and the word's development in this sense is unique to English, though it since has been widely adopted in other languages; perhaps influenced by Latin stupere "be stunned, be stupefied." Intransitive meaning "check oneself" is from 1680s. Meaning "make a halt or stay, tarry" is from 1711. Stop-light is from 1922; stop-sign is from 1918. Stop-motion is from 1851, originally of looms. Related: Stopped; stopping.