Grand slam originally came from the card game of bridge, but the idea of 4 comes from the sport of baseball, where a batter hits a home run with the bases loaded. This means 4 runs are scored on the one swing of the bat. This is the most that can be scored on one play.
So the ball is indeed slammed by the batter, over the fences and out of the playing field. And it is a "grand" play.
The idea that winning 4 tournaments in tennis (or golf or any other sport) is a "grand slam," borrows on the 4 runs scored in the baseball grand slam.
Grand slam (baseball) - Wikipedia, the free encyclopedia
You can even get a "grand slam" breakfast featuring eggs, pancakes, bacon and sausage.
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