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.
early 15c., "relieve (pain); make mild or more tolerable; reduce in amount or degree," from Latin mitigatus, past participle of mitigare "soften, make tender, ripen, mellow, tame," figuratively, "make mild or gentle, pacify, soothe," ultimately from mitis "gentle, soft" + root of agere "to do, perform" (from PIE root *ag- "to drive, draw out or forth, move"). For mitis de Vaan suggests cognates in Sanskrit mayas- "refreshment, enjoyment," Lithuanian mielas "nice, sweet, dear," Welsh mwydion "soft parts," Old Irish min "soft," from a PIE *mehiti- "soft." Related: Mitigated; mitigating; mitigates.