The Impossible Sit Up 

A dirty trick you pull on a friend at a party, where you hold a towel over a friends face while he tries to do a sit up with you pulling down on it. While your friend tries to do this sit up you have another friend bend down over him and pull down his pants then you release the towel and your friends face goes into an asshole.

Kean: Aw man I was so drunk last night but I sobered up really fast when Darren and Matt got me to do The Impossible Sit Up.

The Impossible Sit Up by Matt Walters April 22, 2008

the impossible situp 

when one bounds another across the head with a shirt and tells him to attempt to do a situp while an acomplice drops his pants and points his ass at the person doing the situp, at which time the shirt is released and the one attempting his the ass with his face

Damn, me and jorge got albert so good with that impossible situp.

the impossible situp by Combat 84 June 30, 2004