OK shoot holes in this one
In the days of the Sailing battleships they used to keep the cannon balls in those neat pyrimid shaped piles, to keep them from rolling around the deck. They uses a eggcarton sort of rack to hold the bottom layer in place. This rack was called a Monkey and was typically made from brass. As the temperature would drop the brass would contract enough to pinch the bottom layer of cannonballs out the rack. Hence the term "cold enough to freeze the balls off a brass monkey"