Trivia of the week:
(From my parents.)
A bit of information to help you sleep better now that you know. In the heyday of sailing ships, all war ships and many freighters carried iron cannons. Those cannons fired round iron cannon balls It was necessary to keep a good supply near the cannon, but they had to find a way to prevent them from rolling about the deck. The best storage method devised was a square based pyramid with one ball on top, resting on four resting on nine which rested on sixteen. Thus, a supply of 30 cannon balls could be stacked in a small area right next to the cannon. There was only one problem... how to prevent the bottom layer from sliding or rolling from under the others.
The solution was a metal plate called a "Monkey" with 16 round indentations. But, if this plate was made of iron, the iron balls quickly would rust to it. The solution to the rusting problem was to make "Brass Monkeys." Few landlubbers realize that brass contracts much more and much faster than iron when chilled. Consequently, when the temperature dropped too far, the brass indentations would shrink so much that the iron cannon balls would come right off the monkey.
Thus, it was quite literally, "Cold enough to freeze the balls off a brass monkey".