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."
by Rabadina November 9, 2008
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That threatening feeling you get in the wake of a bowel movement. Typically an urgent feeling which results in immediate vacation of a place, task or conversation that you are in. Threatening bowel movements may strike at inappropriate times. You may experience the intense feeling without prior knowledge that you were anticipating a bowel movement.
Susie: How is your steak, Sharon?
Sharon: uh.. *abruptly leaves*

5 mins later

Susie: Are you okay?
Sharon: Yeah just had a major threatening bowel movement (TBM). But it has been expelled.
by JKF125 June 23, 2010
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Also known as IBMS. A condition affecting many people but widely undiscussed. It creates a sensation of tingling and bloating in the gastrointestinal tract, which then leads to spontaneous bowel movements of which the affected has no control. The bowel movements can vary whether they are consisted of urine, stool or both and they can also vary in quantity.
-I heard you crapped your pants the other day during your presentation.
-It's not my fault, I can't help it...
-How come?
-I don't like to talk about it.
-C'mon, you can let me know, we're best buds!
-Well, okay.....I have Involuntary Bowel Movement Syndrome
-Oh, IBMS.
-How do you know about it?
-My mum's had it for a few years, so I know how you feel.
by lvthmyrkfrv November 11, 2009
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An experience or thing that looks and feels just like taking a crap.
Dude, That protest was so stupid, it was a Bowel-Like Movement (BLM) for sure!
by Tacobaxter November 1, 2020
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