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.
(not to be confused under any circumstances with menstruating, therefore, it's spelled with an a after the m, and not an e): very offensive, for men to be incessantly quetching and complaining about this and that, and pentoute not behaving like a typical pragmatic rational male.
advice for men who hate their boss: stop manstruating about how you hate your boss and don't make enough money, read John Gray's Men are from Mars, women are from Venus, and then behave like a rational, pragmatic male by changing jobs.