Douglas Harper's Etymology Dictionary
1908, "female criminal," second element from nickname of Mary, used of disreputable females since early 1600s; first element from slang gonif "thief" (1885), from Yiddish, from Hebrew gannabh "thief."
A gun moll (aka gangster moll) is the female companion of a male professional criminal. In some contexts, 'gun moll' more specifically suggests that the woman handles a firearm.
When the term came into usage in the first decade of the 20th century, "gun" was not derived from the firearm, but from the Yiddish word meaning "thief," variously transliterated into English as ganefthe, gonif, goniff, or ganof, itself derived from Hebrew "Ganav" (גנב). However, this distinction gradually disappeared, especially when such women became associated with gangsters noted for their frequent use of guns.
"Moll" derives from "Molly", used as a euphemism for "whore" or "prostitute" and attested at least since 17th century England.
In the U.S., the term has mostly been applied to a woman associating with an American gangster of the 1920s and 1930s, and in most cases remarkable only because of his notoriety. Extended use of the term without awareness of the Yiddish root, however, has invited interpretations of "gun" as suggesting more than simply criminal associations. Bonnie Parker and Blanche Barrow were gun molls in this stronger sense, and especially notable examples in general, because of their accompanying the rest of the Barrow Gang to the planned locations of violent crimes, and, in Parker's case, apparently directly assisting at least to the extent of loading guns in the midst of shootouts.
Usage examples of "gun moll".
She spoke from the side of her mouth, a mannerism that gave her the appearance of a Hollywood gun moll.
Great, now the blabbermouth would add gun moll to his list of her talents.