Acronym for the common phrase "just peed my pants while grinning to myself." It is usually used when an individual has literally peed his or her pants after being told a funny joke. Notice it is not laughing and peeing but grinning and peeing. The word is used mainly by people who have mangled vocal cords and also those who generally hate laughter.
Tardy: "Guess what?"
Tardy: "Chicken butt!"
Farty: "JPMPWGTM!!!! THAT'S WUZ SOOOOO FUNNY!!!!! I ACTUALLY CRAPPED MY PANTS TOO HAHAH!"