inexcusable 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.

mid-13c., "attempt to clear (someone) from blame, find excuses for," from Old French escuser (12c., Modern French excuser) "apologize, make excuses; pardon, exonerate," from Latin excusare "excuse, apologize, make an excuse for, plead as an excuse; release from a charge; decline, refuse, excuse the refusal of" (source also of Spanish excusar, Italian scusare), from ex "out, away" (see ex-) + causa "accusation, legal action" (see cause (n.)).

Sense of "forgive, pardon, accept another's plea of excuse" is from early 14c. Meaning "to obtain exemption or release from an obligation or duty; beg to be excused" is from mid-14c. in English, as is the sense "defend (someone or something) as right." Sense of "serve as justification for" is from 1530s. Related: Excused; excusing. Excuse me as a mild apology or statement of polite disagreement is from c. 1600.

late 14c., from un- (1) "not" + excusable. The usual word is inexcusable. Related: Unexcusably.