The saying, "It's so cold out there it could freeze the balls off a brass monkey," comes from the Civil War. In those days, cannonballs were stacked beside cannons in a pyramid formation called a brass monkey. When it became extremely cold outside, these stacked cannonballs would crack and break off.
In the heyday of sailing ships, all war ships and many freighters carried iron cannons. Those cannon 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 thirty 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 sixteen 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 "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!"
There is no better visual cue that it is cold out than this brass monkey. Once the temperature gets to be frigid, about 23 degrees Fahrenheit, a piece of his anatomy drops off as illustrated in the picture above. This will bring life to the adage “cold enough to freeze the balls off a brass monkey.” To reset him all you have to do is let him thaw out a bit and reattach his boys. This may be the only time you find emasculation amusing.