Quiz: Remember the definition of mal de mer?
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)) + pecare "to sin," of unknown origin. Meaning "faultless" is from 1610s. Related: Impeccably.