Origin and history of inapt

mid-14c., "inclined, disposed;" late 14c., "suited, fitted, adapted, possessing the necessary qualities for the purpose," from Old French ate "fitting, suitable, appropriate" (13c., Modern French apte), or directly from Latin aptus "fit, suited, proper, appropriate," adjectival use of past participle of *apere "to attach, join, tie to." This is reconstructed to be from PIE root *ap- (1) "to grasp, take, reach" (source also of Sanskrit apnoti "he reaches," Latin apisci "to reach after, attain," Hittite epmi "I seize"). The elliptical sense of "becoming, appropriate" is from 1560s.

c. 1600, "not fit or suitable, inapt," also "absurd, foolish," from French inepte "incapable" (14c.) or directly from Latin ineptus "unsuitable, improper, impertinent; absurd, awkward, silly, tactless," from in- "not, opposite of" (see in- (1)) + aptus "apt" (see apt). Related: Ineptly; ineptness.

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.