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.
1640s, "pertaining to the senses" apparently coined by Milton to recover the original meaning of sensual and avoid the lascivious connotation that the older word had acquired, but by 1870 sensuous, too, had begun down the same path and come to mean "alive to the pleasures of the senses." Rare before Coleridge popularized it "To express in one word all that appertains to the perception, considered as passive and merely recipient ...." (1814). From Latin sensus (see sense (n.)) + -ous. Related: Sensuously; sensuousness.