unclean (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.

Old English clæne "free from dirt or filth, unmixed with foreign or extraneous matter; morally pure, chaste, innocent; open, in the open," of beasts, "not forbidden by ceremonial law to eat," from West Germanic *klainja- "clear, pure" (source also of Old Saxon kleni "dainty, delicate," Old Frisian klene "small," Old High German kleini "delicate, fine, small," German klein "small;" English preserves the original Germanic sense), perhaps from PIE root *gel- "bright, gleaming" (source also of Greek glene "eyeball," Old Irish gel "bright"). But Boutkan doubts the IE etymology and that the "clean" word and the "small" word are the same.

"Largely replaced by clear, pure in the higher senses" [Weekley], but as a verb (mid-15c.) it has largely usurped what once belonged to cleanse. Meaning "whole, entire" is from c. 1300 (clean sweep in the figurative sense is from 1821). Sense of "not lewd" (as in good, clean fun) is from 1867; that of "not carrying anything forbidden" is from 1938; that of "free of drug addiction" is from 1950s. To come clean "confess" is from 1919, American English.

Old English unclænnes; see unclean + -ness.