When someone reaches down, from behind, to pull another person's underpants up, creating an uncomfortable feeling as it pulls the penis & balls down and up.
An often juvenile prank, that was used by bullies in a school setting, though also common at many adult parties, and college fraternity gatherings.
The 'victim' of a wedgy usually is unsuspecting, and occurs when the 'victim' is bent over.
The prankster reaches down, to grab the waist band, and then will yank hard, pulling the fabric up and squashing the testicles, and wedge the gathered underwear between the butt cheeks.
Some will continue to pull till there is a substantial showing of the underwear, until the person is literally lifted up off the ground or the underwear actually rips.