nut flap

to move your hips up and down in a humping motion so that your scrotum swings up and down giving the appearance of your "nuts flapping".

I saw robert in the hall today doing the nut flap.

by Alden aka Ginger April 14, 2006

nut flapping

Occurs when your testicles smack together when running down the stairs in gym shorts

dude i was so embarassed today i had some major nut flapping going on when i was running down the stairs

by peterbattista November 1, 2006