A "Fat John" is a code word for the act of taking a poop. No one is exactly sure where the name came from, but it originated in Manchester, NH and has since spread. When referring to a Fat John, the you usually pretend it is a real person.
Person 1: "Dude, Fat John is about to visit me!"
Person 2: "No way! He visited me today, and boy, was he angry!"
Person 1: "Aw jeez, I'm about to be visited by Fat John."
Person 2: "Who?"
Person 1: "Everybody's met Fat John. In fact, most people see him everyday. Sometimes he tries to come in before you're ready, and sometimes he can just explode at you. You usually feel better after Fat John leaves."