A cannonball was a solid iron ball that was fired from a cannon using gunpowder. They were used by soldiers and sailors as a missile for inflicting damage to walls and ships from a large distance away. One was fired from Mac the Black's pirate ship directly upward and returned destroying the ship.
Emmett Brown was also fired on by a cannonball which reduced a palm tree to dust.
Behind the scenes
Despite their appearance in animated cartoons, cannonballs do not explode upon impact.