1580s, "not made palatable by seasoning," from un- (1) "not" + past participle of season (v.). Meaning "not habituated by experience" is recorded from c. 1600.
Entries linking to unseasoned
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.
season v.
late 14c., sesounen, "improve the flavor of by adding spices," from season (n.) and from Old French saisonner "to ripen, season" (Modern French assaisoner), from seison, saison "right moment, appropriate time" on the notion of fruit becoming more palatable as it ripens.
Figurative use by 1510s. Of timber, etc., "bring to maturity by prolonged exposure to some condition," by 1540s; hence in extended sense "bring to the best condition or use; of persons "fit to any use by time or habit," c. 1600. In 16c., it also meant "to copulate with." Intransitive sense of "become mature, grow fit for use" is by 1670s.