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1.
The phrase used to describe running whilst needing to excrete one's bowls, the sufferer then has 2 choices:
- clench their arse and hope for the best
- shit their pants
"Paula Radcliffe shit herself in the marathon", "oh yeah we've all had runner's bum"
by eddietheeagle October 12, 2013
 
2.
When you need to shit whilst running and you must clench your asscheeks to hold it in.
- "Hahaha Paula Radcliffe shat herself mid race", "ah, we've all had runners bum"
by Mahoganay October 28, 2013