Bowel Movement (on) Titties: The act of defecating on a woman's breasts prior to, or during the act of sexual intercourse. See Cleveland Steamer.
This acronym is particularly useful in mixed company, or in public.
The phrase developed in a Salt Lake City, UT Subway restaurant when trying to express the attractiveness of one of the cashiers, while not trying to use the phrase "she is so hot, I'd shit on her chest" in public. One of the sandwiches being ordered at the time was the Italian BMT, and the acronym seemed appropriate and was put into use.