When you are boning a girl, and your sack slaps against your grundle (also taint, gooch, or chode).
Dude, I was boning this girl last night, and her brother was awakened by the sound of my grundle smack.
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When you aim to kick someone square in the jaw but miss with your flying front kick, only to hit them square in the face with the area between your nutsack and balls.
Sometimes I think you UD 'volunteer' editor need a good grundle smacking