Tennis player Naomi Osaka won the U.S. Open, her third Grand Slam title. In tennis, the four major tournaments of Australia, France, Great Britain, and the U.S., or winning all of these tournaments is called a "Grand Slam. Not only in tennis, the four major tournaments in golf are also called "Grand Slams," and in baseball, a "Grand Slam" means a grand slam home run. So where did this term come from...