A boston rag is a sexual act between a man and a woman that is currently on her period. The man performs oral sex on the woman and removes the tampon from her vagina with his teeth. He then inserts the tampon in her anus. The woman proceeds to poop out the tampon back into the man's mouth. He reinserts the tampon back in the vagina, this completing the boston rag.

A casual invitation / threat used by an individual that is not afraid of fucking someone up.