1660s, "not made better," from un- (1) "not" + past participle of improve (v.). Sense of "not developed or taken advantage of" (of land) is recorded from 1781.
Entries linking to unimproved
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.
improve v.
late 15c., "to use to one's profit, to increase (income)," from Anglo-French emprouwer "to turn to profit" (late 13c.), from Old French en-, a causative prefix or from em-, + prou "profit," from Latin prode "advantageous" (see proud (adj.)).
Spelling with -v- was rare before 17c.; it apparently arose from confusion of -v- and -u-. Spelling otherwise deformed by influence of words in -prove. Meaning "make better, raise to a better quality or condition" first recorded 1610s. Intransitive sense "get better" is from 1727. Phrase improve the occasion retains the etymological sense. Meaning "to turn land to profit" (by clearing it, erecting buildings, etc.) was in Anglo-French (13c.) and survived or was revived in the American colonies and Australia. Hence, "make good use of, occupy (a place) and convert to some purpose."