a prefix used freely in English and meaning "not, lack of," or "sham," giving a negative sense to any word, 14c., from Anglo-French noun-, from Old French non-, from Latin non "not, by no means, not at all, not a," from Old Latin noenum "not one" (*ne oinom, from PIE root *ne- "not" + PIE root *oi-no- "one, unique"). In some cases perhaps from Middle English non "not" (adj.), from Old English nan (see not). "It differs from un- in that it denotes mere negation or absence of the thing or quality, while un- often denotes the opposite of the thing or quality" [Century Dictionary].
late 14c., "a plug;" mid-15c., "a cessation," from stop (v.). Of mechanisms of musical instruments from c. 1500, especially of organs, where opening them makes it produce more sound, hence figurative phrase pull out the stops (1909). From 1660s in phonetics, 1831 in photography. Meaning "a stopping place" is from 1889. To put a stop to some activity is from 1670s (earlier give a stop to, 1580s).