perfectly adv.

early 15c. classical correction of Middle English parfit "flawless, ideal" (c. 1300), also "complete, full, finished, lacking in no way" (late 14c.), from Old French parfit "finished, completed, ready" (11c.), from Latin perfectus "completed, excellent, accomplished, exquisite," past participle of perficere "accomplish, finish, complete," from per "completely" (see per) + combining form of facere "to make, to do" (from PIE root *dhe- "to set, put").

Often used in English as an intensive (perfect stranger, etc.), from the notion of "complete." Grammatical sense, in reference to verb tense describing an action as completed, is from c. 1500. As a noun, late 14c. ("perfection"), from the adjective.

The difference between the Preterit and the Perfect is in English observed more strictly than in the other languages possessing corresponding tenses. The Preterit refers to some time in the past without telling anything about the connexion with the present moment, while the Perfect is a retrospective present, which connects a past occurrence with the present time, either as continued up to the present moment (inclusive time) or as having results or consequences bearing on the present moment. [Otto Jespersen, "Essentials of English Grammar," 1933]

common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). 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.