Abbreviation for the words Blast From the Ass. BFA is the act of having an absolutely terrible diarrhea attack that exploads from your buttocks as you sit down to the toilet seat. A polite code name to use instead of saying, "Oh, no! I have to take a shit!" You can use the term so others in your group/significant others know there is an emergency without notifying strangers.
You're in a club surrounded by hot people who you don't want to know that you need to take a massive shit. "Oh, man! BFA on board!"