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A phenomena occurring during gay sex where the giving man's scrotum slaps the receiving man's scrotum, sending them forward to the grundle then back to hitting the giver's scrotum, which then swing backwards, and repeats.

This is derived from the desktop toy known as Newton's balls
Hannable was riding Adam so hard that the Newton's Grundle could be heard from outside the bathroom.
by horribleperson24601 April 15, 2014
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