Stair Clap 

The clapping of a male's scrotum against his inner thigh when he runs down or maybe even up stairs.

When Tommy ran down stairs excitedly, he Stair Clapped and got sick from the impact.

Stair Clap by schaeferqwaefer November 22, 2009

Stairclap 

When you're running down the stairs very quickly in boxer shorts, and the weight of your testicles causes your scrotum to move against your gooch and make a loud clapping sound.

John: Ew, Mike. Yesterday I was running down the stairs and I did a stairclap. My mom heard. She looked grossed out.
Mike: EWW! DUDE, I DIDN'T NEED TO KNOW YOU HAVE A NASTY STAIRCLAP!

Stairclap by unknown dropouts April 15, 2011