A gangster's girlfriend or female companion, or historically, a woman who associates with criminals.
From the name 'Moll,' a common name for women in the 17th century, particularly those of lower social status; came to be slang for a criminal's female associate.
The term 'moll' peaked during American Prohibition—movies and newspapers made gangsters' girlfriends almost as famous as the criminals themselves, creating an outlaw mystique.
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