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.