The opposite of a wedgie, usually inflicted on oneself accidentally. When sat on the toilet and you stand up pulling up your pants but leaving your underpants around your knees.
Also self-inflicted in times of panic, as when your pants are down as your girlfriend is giving you a blow job and her parents come home. In a panic you pull up your pants, again leaving your underpants around your knees.
"Jenny's parents came home while she was giving me a blow job, right there in view of their hallway. I reacted to fast I gave myself a reverse wedgie."