Short for Post Jizz Piss. The piss you take after cumming when some of the jizz is still hanging around and you piss it out. This normally is accompanied with an uncontrolled direction of piss spraying in random directions. Usually a roomate can spot the jizz in the toilet.
Roomate: Dude, you got your PJP all over the seat!
On a message board, a sad little man named PJP(which stands for "Penis Jockey Pederast") got some of his posts deleted. It caused him to have a nervous breakdown. The term is used when somebody causes another person to have a psychotic episode for silly reasons.