female proper name, shortened form of Mollie, Molly, itself a familiar of Mary. Used from c. 1600 for "prostitute," but in low slang by early 19c. it also meant "female companion not bound by ties of marriage, but often a life-mate" [Century Dictionary]. It became a general word for "woman" in old underworld slang, for instance Moll-buzzer "pickpocket who specializes in women;" Moll-tooler "female pick-pocket." U.S. sense of "a gangster's girlfriend" is by 1923.

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Definitions of Moll from WordNet

moll (n.)
the girlfriend of a gangster;
Synonyms: gun moll / gangster's moll