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.
"deserving censure, blameworthy," late 13c., coupable, from Old French coupable (12c., Modern French coupable), from Latin culpabilis "worthy of blame," from culpare "to blame," from culpa "crime, fault, blame, guilt, error." De Vaan writes that this might be from a PIE root *kuolp- "to bend, turn" (source also of Greek kolpos "bosom, lap;" see gulf (n.)). According to his sources, "The original meaning of culpa is 'a state of error' rather than 'an error committed'." English (and for a time French) restored the first Latin -l- in later Middle Ages. Related: Culpably; culpableness.