When performing anal sex, the male pulls out a chili dog with undigested corn in the batter - the friction from churning butt-butter pops the corn kernels creating a popcorn dog.
After pulling out of Roy's ass, Siegfried was surprised to see the corn in the shit on his dick begin to pop from the friction, creating the mythical popcorn dog.
The crunchy bits left after an average person is done with their popcorn. Including but not limited to: burnt bits, half-popped kernels, shells, and un-popped kernels.
Scott scrounged the popcorn dregs out of a bag left over from last night's party.