Origin and history of impertinently

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.

common adverbial suffix, forming, from adjectives, adverbs signifying "in a manner denoted by" the adjective; Middle English -li, from Old English -lice, from Proto-Germanic *-liko- (source also of Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko). See -ly (1). It is cognate with lich, and identical with like (adj.).

Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.