HISTORY OF BRASS BALLS
Once again we aim to upgrade your education with historical information that you probably didn't hear in school.
In the once and mighty British Navy, in the time of their Empire building, every sailing ship had cannon (the plural of cannon) for protection. While they ruled the seas, they still had to deal with pirates and upstart nations. Thus the cannon.
Cannon of the times required round iron cannonballs. A ship's master wanted to store the cannonballs such that they could be available for instant use when needed, but in a manner that would not let them roll around the gun deck.
Being practical minded it didn't take the sailors manning the guns to find a solution. And that was to stack them up in a square-based pyramid next to the cannon. The top level of the stack had one ball, the next level down had three, the next had nine, the next had sixteen, and so on. Four levels would provide the gunners 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 brass plate referred to as a "brass monkey", with one rounded indentation for each cannonball in the bottom layer.
Brass was used because the cannonballs wouldn't rust on the brass monkey, a problem with all the sea spray and wet air on a sailing ship. And this proved to work very well, except when the weather became cold.
As it got cold on the gun decks, the indentations in the brass monkey would shrink and become smaller than the iron cannonballs they were holding. If the temperature got cold enough, the bottom layer of cannonballs would pop out of the indentations, thus spilling the entire pyramid over the deck.
Hence the term, quite literally, it was "cold enough to freeze the balls off a brass monkey."
And so, another familiar phrase became part of everyday language.