A sneaky term for sodomy generaly used by pedophelic priests to trick young boys of the church to submit to anal sex. This term refers to anal sex and the origin of the name is likely the fact that sodomy and the excretion of feces are similar except for the direction of movement of the particular object residing in the rectum.
Father Brown: "Young man, your sins against the church, and against God are without number. Those were terrible acts, and there is only one way for you to repent. We must perform... The Reverse Poo. And when that's finished, we can play a nice game of Suck-the-Banana."