What does blind sit-up mean?

blind sit-up meaning in Urban Dictionary

To do a sit-up along with your eyes sealed. Usually a prank in wich one party bets another party they cannot do a sit-up with their eyes closed. As soon as the 2nd celebration attempts to finish this the first celebration brings straight down their jeans and locations their buttocks where the face of 2nd celebration will be upon doing the sit up. This always ends up in hilarity for the very first celebration and a brown nostrils when it comes to 2nd.