Cold enough to freeze the balls off a brass monkey

The "monkey" in this saying is thought not to be paying reference to our ape friends. The "Brass Monkey" was a square brass plate set into the wooden deck of early warships to hold cannonballs. These cannonballs would be stacked in a pyramid shape ready for use. When the temperature dropped significantly the metal would contract and thus disturb the carefully stacked cannon balls causing them to collapse from their pyramid shape.