Throwing in the bowel
Referring to "throwing in the towel", this is when you really have to take a dump, and when you just can't wait any longer, you throw in the bowel, giving up on your body, and taking a crap in your pants.
Oh, when I took all those laxatives, I was in the middle of a meeting. I couldn't take it any longer, so I did the old 'throwing in the bowel' routine.