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1.
when youre banging a fat chick and your nuts slap your grundle then her taint so furiously that it makes a boom boom clap sound.the reason the sound is boom boom clap is because one boom for you one fer her and then the clap is the sound of her thighs smacking together.
neil was banging this fat chick so hard that all that his buddies next door heard was grundle thunder. Neil then walked out with bloody ears when he exeperienced sonic boom.
by ivcabel August 07, 2006
 
2.
When someone is rubbing your grundle or sack area intensly, and you chafe to the point of bleeding and then you let out very thunderous bloody shart onto your partners face.
Peter got major grundle thunder from brandi last night, he came home bloody and smelling of shart
by Peter Boardman October 08, 2005