When you take a particularly large, and thus painful, shit. A shit that gives you the feeling of a gaping wide asshole and/or a rip in it or the passageway leading to it, and also would give you trouble walking straight for a while. Similar to the feeling you would get whilst getting fucked up the ass by a big black dick in prison. Essentially, a shit that is equal to the size of a black dick.
I had some of George's awesome homemade chili that his mom made for him, the one with mystery meat. Not a good idea, I ended up having to take a prison shit.