1530s, from un- (1) "not" + past participle of bleach (v.).
Entries linking to unbleached
un- 1
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.
bleach v.
Middle English blechen, from Old English blæcan, of cloth or fabric, "to make white by removing color, whiten" (by exposure to chemical agents or the sun), from Proto-Germanic *blaikjan "to make white" (source also of Old Saxon blek, Old Norse bleikr, Dutch bleek, Old High German bleih, German bleich "pale;" Old Norse bleikja, Dutch bleken, German bleichen "to make white, cause to fade"), from PIE root *bhel- (1) "to shine, flash, burn," also "shining white."
The same root probably produced black (q.v.), perhaps because both black and white are colorless, or because both are associated in different ways with burning. For the contrary senses, compare Old English scimian, meaning both "to shine" and "to dim, grow dusky, grow dark," which is related to the source of shine.
The intransitive sense of "become white" is from 1610s. Related: Bleached; bleaching. The past participle in Middle English was sometimes blaught.