"not capable," 1590s, from French incapable (16c.) and directly from Medieval Latin incapabilis "incapable," from in- "not" (see in- (1)) + capabilis "receptive; able to grasp or hold" (see capable). Related: Incapably; incapability.
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.
"sufficiently able, having power or capacity, qualified," 1590s, from French capable "able, sufficient; able to hold," or directly from Late Latin capabilis "receptive; able to grasp or hold," used by theologians, from Latin capax "able to hold much, broad, wide, roomy;" also "receptive, fit for;" adjectival form of capere "to grasp, lay hold, take, catch; undertake; take in, hold; be large enough for; comprehend" (from PIE root *kap- "to grasp"). Other late 16c. senses in English, now obsolete, were "able to comprehend; able to contain; extensive." Related: Capably.