imperfect (adj.)

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]

late 14c., "incompleteness, deficiency, lack," from Old French imperfeccion "defect; imperfect state" (12c.) and directly from Late Latin imperfectionem (nominative imperfectio) "imperfection," noun of action from Latin imperfectus "imperfect"(see imperfect). Meaning "an instance of being imperfect" is from early 15c.

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.