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When a woman masturbates while she's having her period. The word is coined from the words "masturbation" and "menstruation(i.e. when a woman has her period).
When the voyeur caught sight of the black-haired, pale-skinned goth chick lying naked and menstrubating on a towel on the floor, he immediately thought to himself, "That is one sick bitch!"

Mark H. Expanding the sexual slang vocabulary since February 2004.
by Mark H October 10, 2004
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