Wedgie

The practice of forcing the gusset of one's (or another's) underwear between the buttocks to create a visible cleft. Particularly amusing if inadvertantly caused by high waisted trousers. Walking with a wedgie is "Chewing cheese".
Variations include Skidders McKenzie, based on the gold watches (q.v.) you would gain from a severe wedgie

Also known as a Melvin. (See: Bill and Ted etc etc)

Actually, a melvin was in the front, and a wedgie was in the back. A particularly feared prospect was the Atomic Melvin, in which one's stretchy boxer-briefs were pulled so far up that the elastic band could be placed over one's head.