Imperfect

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Middle English imparfit, imperfet, from Old French imparfait, from Latin imperfectus, from in- ‘not’ + perfectus (see perfect). The spelling change in the 16th century was due to association with the Latin form.

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From Middle English imperfit, from Old French imparfit (modern French imparfait), from Latin imperfectus. Spelling modified 15c. to conform Latin etymology. See im- +‎ perfect.

etymonline

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imperfect (adj.)

late 14c., imparfit, "sinful, immoral; not properly formed, not complete, immature; rudimentary, elementary," from Old French imparfait, from Latin imperfectus "unfinished, incomplete, immature," from assimilated form of in- "not, opposite of" (see in- (1)) + perfectus "finished, complete" (see perfect (adj.)). Altered mid-16c. to conform to Latin, along with perfect. Related: Imperfectly.