When you are trying to poo (shit) but the faeces gets lodged in the opening to the anal cavity, often known as a turtle's head poking out. When you can push no more, but the turtle remains trapped, wiping during this scenario is known as Wiping the Turtle's mouth.
Craig came back from the dunger and informed Jon "That was a tough one Jon, I just ended up wiping the turtle's mouth" Jon understood instantly.