Origin and history of impertinent

late 14c., "unconnected, unrelated, not to the point" (now obsolete; OED's last citation is from Coleridge), from Old French impertinent (14c.) or directly from Late Latin impertinentem (nominative impertinens) "not belonging," literally "not to the point." This is from assimilated form of Latin in- "not, opposite of" (see in- (1)) + pertinens (see pertinent).

The sense of "rudely bold, uncivil, offensively presumptuous" is from 1680s, from an earlier sense of "not appropriate to the situation" (1580s), which probably is modeled on similar use in French, especially by Molière, from notion of meddling in what is beyond one's proper sphere.

Impertinent means forward, intrusive, generally from curiosity but sometimes with undesired advice, etc.; officious means forward to offer and undertake service where it is neither needed nor desired. A busybody may be either impertinent or officious, or both. [Century Dictionary]

Rare alternative unpertinent is attested from c, 1400.