A pantsplosion occurs when you shit yourself with such violent ferocity that feces erupts from the beltline of your pants. This is usually the result of being incontinent when wearing tighty-whities (the shit just has nowhere else to go).
Person1: Dude, what the hell? Your back is covered with shit!
Person2: Yeah, I ate too many rancid gas station hot dogs and couldn't make it to the can before pantsplosion.
Person2: Dude, it's up to your shoulder blades?