1540s, from un- (1) "not" + past participle of prepare (v.).
Entries linking to unprepared
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.
prepare v.
mid-15c., "set in order or readiness for a particular end," a back formation from preparation and in part from Old French preparer (14c.), from Latin praeparare "to make ready beforehand," from prae "before" (see pre-) + parare "to make ready" (from PIE root *pere- (1) "to produce, procure"). Compare pare (v.), which is from the same Latin verb. Related: Prepared; preparer; preparing.
Intransitive sense of "make (oneself) ready beforehand" is from c. 1500. The sense of "bring into a particular mental state with reference to the future" is by 1520s. The sense of "make (food) ready to eat" is from late 15c. (Caxton). The meaning "provide or procure for future use" is from 1530s. An earlier verb was preparate (late 14c.), from Latin praeparatus, past participle of praeparare. The Boy Scouts' motto Be Prepared is attested from 1911, based, as he said, on the initials of the organization's founder, Robert Baden-Powell.