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1580s (earlier complice, late 15c.), from Old French complice "a confederate," from Late Latin complicem (nominative complex) "partner, confederate," from Latin complicare "fold together" (see complicate). With parasitic a- on model of accomplish, etc., or perhaps by assimilation of indefinite article in phrase a complice.
in law, a person who becomes equally guilty in the crime of another by knowingly and voluntarily aiding the other to commit the offense. An accomplice is either an accessory or an abettor. The accessory aids a criminal prior to his crime, whereas the abettor aids him during the act itself.