This is what I have been told, and this is a tidbit of info relating to it:
The phrase Cold enough to freeze the balls off a brass monkey came from the early 1700′s when sailors would stack iron cannon balls on a brass platform called a Monkey
When the weather would turn really cold and the temperature would drop drastically the brass monkey metal would contract more than the iron cannon balls and the cannon balls would roll off the brass monkey platform.
Thus the phrase Cold enough to freeze the balls off a brass monkey was originated.