HEY SAILOR!
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 how 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/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 would quickly rust to it.
The solution to the rusting problem was to make the "Monkeys" out of brass.
Few landlubbers realize that brass contracts and expands more and 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!"
(And all this time, you thought this was an improper expression, didn't you?) ;-)