inadvertence (n.)

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.

early 15c., advertisen, "to take notice of" (a sense now obsolete), from Old French advertiss-, present-participle stem of advertir (earlier avertir) "make aware, call attention, remark; turn, turn to" (12c.), from Latin advertere "to direct one's attention to; give heed," literally "to turn toward," from ad "to, toward" (see ad-) + vertere "to turn" (see versus).

The transitive sense of "give notice to others, inform, warn; make clear or manifest" (mid-15c.) is by influence of advertisement; the specific commercial meaning "call attention to goods for sale, rewards, etc." emerged by late 18c. Compare advert (v.) "turn (someone's) attention to."  Related: Advertised; advertising.

1650s, "not properly attentive" (of persons), a back-formation from inadvertence. Meaning "unconscious, unintentional" (of actions) is from 1724.