The phrase, “cold enough to freeze the balls off a brass monkey” comes from the times of old warships.
The “balls” refer to cannonballs that were made of iron. They were kept on a square brass tray called a “monkey”. In very cold weather, the brass would contract in size, so a square pyramid of cannonballs that was made on it would literally fall off. The reason they were kept on brass was the fact that iron rusts very fast, and it wouldn’t be a good idea to have your cannonballs rusting into the thing they were lying on.
Curiosities from the web, presented by Romano Pisciotti