When your having sex with a girl and she starts menstruating and you pull out after leaving a load, and the semen and menstrual fluids start coming out and mixing together making this dirty ass goop.
Also used after calling a guy, usually Korean, Semen. When his name is Simon. It is to take it to a 2nd level of name morphing
Sebastian - yo Semen
Simon - fuck you Sebastard
Sebastian - easy there semenstruation
Simon - i was totally banging this chick when i felt something weird so i pulled out after letting a load in. she semenstruated everywhere.