- un- (1)
- prefix of negation, Old English un-, from Proto-Germanic *un- (cf. Old Saxon, Old Frisian, Old High German, German un-, Gothic un-, Dutch on-), from PIE *n- (cf. Sanskrit a-, an- "not," Greek a-, an-, Old Irish an-, Latin in-), comb. form of PIE root *ne "not" (cf. Avestan na, Old Church Slavonic and Lithuanian ne "not," Latin ne "that not," Greek ne- "not," Old Irish ni, Cornish ny "not"). Often euphemistic (e.g. 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 (e.g. 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-; cf. 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.
- un- (2)
- prefix of reversal, deprivation, or removal (e.g. unhand, undo, unbutton), Old English on-, un-, from Proto-Germanic *andi- (cf. Old Saxon ant-, Old Norse and-, Dutch ont-, Old High German ant-, German ant-, Gothic and- "against"), from PIE *anti "facing opposite, near, in front of, before, against" (see ante).
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)).