An unfortunately common misspelling of the more accurate term "menstruating". Quite popular on UrbanDictionary.com
Examples abound, far too many to begin to list here. "Mensturating" typically appears to feature in colorful, often crude, articles written by juveniles more interested in the 'gross factor' of their articles than the accuracy of their spelling.
by Gleaming cat-eye April 16, 2010
buy the domain for your art blog