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-14c., reproche, "a rebuke, blame, censure" directed against a person; also "object of scorn or contempt;" c. 1400, as "disgrace, state of disgrace," from Anglo-French repruce, Old French reproche "blame, shame, disgrace" (12c.), from reprochier "to blame, bring up against."
OED cites Diez for the explanation that this is from Vulgar Latin *repropiare, from Latin re- "opposite of" + prope "near" (see propinquity), with suggestions of "bring near to" as in modern get in (someone's) face. But it points out other etymologists of French would have it from *reprobicare, from Latin reprobus/reprobare "disapprove, reject, condemn" (see reprobate (adj.)).
<a href="https://www.etymonline.com/word/irreproachable">Etymology of irreproachable by etymonline</a>
Harper, D. (n.d.). Etymology of irreproachable. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/irreproachable