incompetent (adj.)

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.

late 14c., "suitable, answering all requirements, sufficient, adequate," from Old French competent "sufficient, appropriate, suitable," and directly from Latin competentem (nominative competens), present participle of competere "coincide, agree" (see compete). It preserves the classical Latin sense of the verb, whereas the meaning in compete is a post-classical evolution. Meaning "able, fit, having ability or capacity" is from 1640s. Legal sense "having legal capacity or qualification" is late 15c. Related: Competently.

1610s, from incompetent + abstract noun suffix -cy.