When you go so deep into the pussy that you go beyond balls deep. In this situation, a mans testicles actually enter the vaginal cavity, and he is then "up to his grundle" (the taint) in her vagina. Essentially, his testicles become the functional equivalent of Volvo crash impact dummies against her cervical membrane.
Yo son, I hyperthrusted this broad and ended up grundle deep. She screamed out that she wished she had a cervical air bag!
by Roscoe Sanchez April 09, 2008
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