"foolish," mid-15c., from Latin insipientem (nominative insipiens) "unwise, foolish," from in- "not" (see in- (1)) + sapientem (see sapient). "Now mostly, or wholly, disused to avoid confusion with incipient" [OED].
word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."
In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.
"wise," late 15c. (early 15c. as a surname), from Old French sapient, from Latin sapientem (nominative sapiens), present participle of sapere "to taste, have taste, be wise," from PIE root *sep- (1) "to taste, perceive" (source also of Old Saxon an-sebban "to perceive, remark," Old High German antseffen, Old English sefa "mind, understanding, insight").