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The urgent evacuation of one's bowels due to the increase in intra-abdominal pressure following a large meal. Also refered to as P4.

i.e. When you eat so much that you have to go to the toilet right away just to reduce the pressure.
"I've just eaten too much, I think I need to do a P4."
"What's that??"
"A Postprandial pressure poo!"
by DR_GP August 20, 2011
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