unaffected adj.

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.

late 14c., "mental state," from Latin affectus "disposition, mood, state of mind or body produced by some external influence," noun use of adjective affectus "disposed, constituted, inclined," literally "furnished, supplied, endowed," past participle of afficere "to do; treat, use, manage, handle; act on, do something to; attack with disease; have influence on, apply force to." This Latin verb, used of many different actions, is literally "to do to," from ad "to" (see ad-) + facere (past participle factus) "to make, do" (from PIE root *dhe- "to set, put"). The noun affect seems to have been obsolete outside of psychology, where it is a modern coinage, translating German Affekt. Related: Affects.