To do a sit-up with your eyes closed. Usually a prank in wich one party bets another party they cannot do a sit-up with their eyes closed. When the second party tries to complete this task the first party pulls down their pants and places their buttocks where the face of the second party will be upon completing the sit up. This always results in hilarity for the first party and a brown nose for the second.
Aww i can't believe dave did a blind sit-up!