Poop without warning. You're walking down the street, half a mile from the nearest known bathroom and suddenly its there.
Characterized by potent farts that offer temporary relief, and wave after wave of the poop charging your anus, then backing off for a brief minute.
Frequently ends in fart-moist boxers, hurriedly slamming a bathroom stall door, and finally, release.
I was walking back from an all-you-can-eat buffet when I got thunderpoop and nearly had to run back to the dorm.