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When you evacuate your colon so fast that the intestinal walls slap together and cause pain.

The condition usually occurs after consuming bulk fiber or Pho noodles that act like a sort of turd rebar. The vacuum created by eliminating this exceptionally long and fat fiber dumpling at high speed causes the colon walls to collapse and impact each other.
You: Boss, I have to leave work because I've just had an instance of colon slap.

Boss: Okay, have a craptastic day and cut back on the Pho noodles and bulk fiber.
by Capt Craptastic May 13, 2010