A cannonball is a heavy, round metal ball that was fired from cannons in warfare from the 1400s through the 1800s. These iron spheres, usually weighing between 6 and 40 pounds, could smash through wooden ship hulls, knock down castle walls, or tear through groups of soldiers. Cannons and cannonballs revolutionized warfare because they could destroy fortifications that had been considered impregnable for centuries.

When a cannonball hit something, it didn't explode like bombs do in movies. Instead, it caused damage through sheer weight and speed, like being hit by a very fast bowling ball. Naval battles during the Age of Sail often involved ships firing cannonballs at each other until one vessel was too damaged to continue fighting.

The word also describes a particular way of jumping into a pool: you tuck your knees to your chest and wrap your arms around them, making your body as round and compact as possible. When you hit the water, you create a huge splash. Kids yell “Cannonball!” as they jump, partly because it's fun to say and partly because you really do make an impact similar to something heavy dropping into the water. A well-executed cannonball can soak everyone standing nearby, which is often precisely the goal.