If we define the bases in sex as:
1 - French
2 - Feel
3 - Finger
4 - Fuck
then it stands to reason that instead of calling it 4th base, we would say a "Home Run" if it happened in one night.

Then, in baseball terms a "Grand Slam" is a "Home Run" that scores 3 additional points or "Runs", it stands to reason that a "Grand Fuck Slam" would be a "Home Run" that scores 3 additional points or "Runs."

In colloquial terms: a Grand Fuck Slam is having sex with the same partner 4 times in one night, where both partners reach their climax each time.
Steven went out one night to a party and brought home the hottest girl and proceeded to have the time of their lives. At 1am they finished the first round; both excited and energetic from the party, all they required was a quick water break before hoppinng back in the sack for a second adventure. 20 minutes later both of them finished and needed a little longer break. After ordering pizza which arrived at 2:15am, the two were horny as can be and had sex again. This time, they spilled the pizza off the bed by accident. Just as Steven was getting ready to fall asleep, his party trophy decided to go down on him for an invitation for a fourth round. Finally, 35 minutes later, both were extremely exhausted and collapsed onto Steven's bed in a puddle of sweat and bodily fluids. The excercise had physically drained both of them to the point of literal exhaustion.

Awakening the next day, Steven found that his girl was gone, but the puddle was still around him in the bed. When he went to do his laundry his roommate accosted him of being loud all night long. "You would be too if you had a Grand Fuck Slam with a girl," he replied.

The roommate said, "I don't even know what that is, but I take it you got laid last night."

"You're damn right I did!" Steven responded, "Four times in one night!"
by humor4fun April 29, 2014
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