GRAND SLAM sent by an ESL teacher, an old American friend, Michael Mingucci, who took the time to explain this expression. He's also a baseball fan.
"Grand Slam" is originally a baseball term. I can't explain the whole complicated game here, but basically there are 3 bases that one has to run around to get back to "Home" where he started. When one does this successfully, usually with help from his team mates, he scores 1 point. Each point scored is called a "run." If a player is able to go all the way around the bases at one time, on his own, it is called a "home run." However, if there is already 1 player on each of the 3 bases and the batter hits the ball out of the park, 4 runs are scored. This is a grand slam. THEREFORE, "grand slam", figuratively, can be something that has been done very, very successfully. Use it sparingly, if at all. :-)