IDIOMS

pinch one off

(idiomatic) to defecate

Example: 2006, Mystery of the Urinal Deuce
  You got a whole school here Clyde! M'kay?, you got over 300 people that need to use the boys room!, and you decide you're gonna be a comedian M'kay?, and pinch one off in the urinal! and leave it laying there for everyone to look at!