aka SBM, when you think you are just going to the bathroom to take a piss, and you unexpectedly have the urge to poop.
"Dude, what took you so long to piss? Did you fall in?!"
"Nah man, I had a Surprise Bowel Movement."
"Nah man, I had a Surprise Bowel Movement."
by Rabadina November 9, 2008