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.
"containing much in comparatively small limits," 1610s, from French comprehénsif, from Late Latin comprehensivus, from comprehens-, past participle stem of Latin comprehendere "to take together, to unite; include; to comprehend, perceive" (to seize or take in the mind), from com "with, together," here probably "completely" (see com-) + prehendere "to catch hold of, seize," from prae- "before" (see pre-) + -hendere, from PIE root *ghend- "to seize, take." Related: Comprehensively (mid-15c.); comprehensiveness.