urban legend. fictitious homosexual pervert who preys on young boys. name originates from his favorite action to perform on his victims.
two boys in the gym shower after playing sports
tom: "oh shit, i think i cut my dick up."
henry: "you better cover that up, here comes manjack."
A turd so large that it contacts the bottom of the U-bend while still emerging from the unfortunate crapper's anus, causing him/her to be literally jacked up off the seat. There is a particular risk of manjack when using toilets of the curious German or Austrian design, which have the U-bend at the very front of the pan and a shallow "deck" for inspection of stools.
"How long are you going to be in there?"
"Sorry, got a bad case of manjack and I've run out of toilet paper".