Wiktionary
n. 1 A criminal accomplice 2 (context informal English) A close associate of another
WordNet
n. someone who assists in a plot [syn: confederate, collaborator, henchman]
Usage examples of "partner in crime".
Her partner in crime was pouring some sort of pink foam into a wide pitcher.
With trembling hands, Ruby Bee unlocked the door and slipped in like a tubby bolt of greased lightning, her partner in crime on her heels.
She'd never been the same since my occasional partner in crime Shotgun Suzie introduced her to the old Avengers TV show.
And it may be as well to state here that in due course of time Jesse Pelter and his partner in crime, Grimes, alias Haywood, were tried and sentenced to long terms in prison.
MacFinn's partner in crime, killing on the off nights to keep suspicion from falling on him?