Short sheeting is a prank use when you rig a person's bed, here's how. First they have to have more than one blanket or sheet (one on the top and one on the bottom). Take the one on the bottom and pull out the part of the sheet out of under the bottom part of the mattress and tuck the top part of the sheet under the top part of the mattress. Then fold the bottom half of the sheet to look like it's still the way it was before. When they get in the bed they think their legs will stretch to the end of the bed like normal but little do they know the bed has been short sheeted and they will not be able to get their legs to the bottom of the of the bed because the folded part of the sheet will be in the way.
"Amy poured water on me in my sleep!"
"Let's short sheet her bed."
To fold and tuck in the top sheet of (a bed) so that it simulates both the top and bottom sheets: when the victim of this joke enters the bed, his or her legs are obstructed by the fold.
Man 1: Hey, I shortsheeted my friend in his bed!
Man 2: Really?... O.o? Do you hear that?
Friend: *muffled scream*
Man 1: Uh oh...