When pissing outdoors and the perceived threat of an insect, animal, or person makes you jump back causing your piss to fly everywhere, often times including on yourself.
(Joe returns to the car stopped on the side of the highway.)
Mark: Oh man, d'you piss all over your shorts?!?
Joe: Ugh.. no.
Mark: Did a deer startle you while you took a leak or what?
Joe: No, uhm, a butterfly.
Mark: Dude, you're such a pisshopper!