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

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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.

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