Common mostly with females, a surprise poo occurs when you go to the toilet to have a piss but end up following through- fortunately on the toilet- as you were unaware of your bowel evacuating needs.

Uncommon with males as they pee standing up.
girl 1: I've been waiting 10 minutes for you, where've you been??
girl 2: I had a surprise poo, sorry
girl 1: Oh, okay

Eavesdropping dude: wtf
by bricksatmyface December 16, 2012
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When you go to the bathroom to pee but all of a sudden you need to poop. This is a ‘poo surprise’.
Person A - Hey man I just gotta go pee.
20mins later...
Person B - Haha you had a poo surprise didn’t you!?
by Supersparkleunicorn December 17, 2018
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