1530s, "not capable of sin," from Middle French impeccable (15c.) or directly from Late Latin impeccabilis "not liable to sin," from assimilated form of in- "not, opposite of" (see in- (1)) + peccare "to sin" (see peccadillo). Meaning "faultless" is from 1610s. Related: Impeccably; impeccant; impeccancy.
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