A well trained road rallyist, who enjoys speeding on the open road. A driver with complete disregard for authority. A phrase made popular by Brock Yates, creator of the original "Cannonball Run" aka Cannonball Sea to Shining Sea Memorial Dash (1971).
"Look at that crazy nut on the road, he thinks he's a real cannonballer!"
Used to describe taking a dump with such velocity that your ass is splashed with water, resembling a cannonball hitting the water. Typically a result draft beer and fast food.
I went to the bar last night then ate a quarter-pounder, then birthed a cannonballer this morning.