Used to describe taking a shit. Used more so to describe taking a dump where a BIG piece of shit is "pinched". The BIG piece of shit is the "loaf" as in a loaf of bread in terms of size. To pinch a loaf is to shit out a turd that is the size of a loaf of bread. Pinching a loaf usually involves extreme stretching of the anus and pain is involved in pinching the loaf.
"I feel a huge turd coming on so I have to go pinch a loaf."