In the heyday of the Sailing ship, every ship had to have a cannon for protection. Cannon of the times required round iron cannon balls. The master wanted to store the cannonballs such that they could be of instant use when needed, yet not roll around the gun deck.
The solution was to stack them in a square-based pyramid next to the cannon. The top level of the stack had one ball, the next level down had four, the next had nine, the next had 16, and so on. Four levels would provide a stack of 30 cannonballs.
The only real problem was how to keep the bottom level from sliding out from under the weight of the higher levels. To do this, they devised a small plate (“monkey”) with one rounded indentation for each cannonball in the bottom layer .
When iron was used to make this plate (“monkey”) the cannonballs would rust to the plate. As a result, these plates were made of brass to prevent this problem-thus the “brass monkey.”
When temperature falls, brass contracts in size faster than iron. As it got cold on the gun decks, the indentations in the brass monkey would get smaller than the iron cannonballs they were holding. If the temperature got cold enough, the bottom layer would pop out of the indentations spilling the entire pyramid over the deck.
Thus it was, quite literally, “COLD ENOUGH TO FREEZE THE BALLS OFF A BRASS MONKEY.”
I have heard of a product that is a statue of a monkey you put in your yard. I don’t think it is real brass, but it is at least colored to look like brass. The statue is anatomically correct and, when it gets below freezing, the piece between the monkeys legs falls off. When the temperature warms, it can be reattached.