**A Brass Monkey on a ship is a brass or bronze object shaped like a flat box or tray which once served as a device to store and transport cannonballs on a warship. ** It reportedly originated in the age of sailing ships, as the flat treatment of the brass prevented the cannonballs from rolling around as cannons rocked from side to side.
The phrase “cold enough to freeze the balls off a brass monkey” likely originated as a reference to this device, as cold conditions would cause the metal to contract and release its grip on the cannonballs.