Pinching a loaf refers to when one's sphincter clenches up mid-bowel movement, and cuts the egressing turd in half. The result is usually that one will have to wipe oneself raw in an effort to return one's rectum to a pristine un-befouled state of cleanliness.
Wife: Honey, what's taking you so long. We're going to be late.
Husband: I pinched a loaf, dear. Now I'll have to take another shower.
Wife: Oh, the humanity! I'll see if I can push back our reservations.