To push one's fist right into another's asshole. This is also called an JAM UP. Normally used in times of extreme agressiveness. Males in particular detest this as it enlarges the rectal passage by up to six times.
A: Did you see Tom yesterday?
B: No, he skipped school to miss the maths test.
A: I could give him a fucking fist up.
B: Here, let me give you one, you foulmouthed bastard.
A: MOM?!?! NO NO NO! AAAAAAAAAAAAARGH!!!