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.
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