When it is cold enough to Freeze the Ball’s off a Brass Monkey, we really had better wrap up warm. This expression has long puzzled me because I couldn’t connect brass monkey statues with balls that drop off in freezing cold weather. Who ever heard of such a thing? So I searched through the dusty old nautical records and eventually found the answer. The guns on 18th century men of war ships needed gunpowder to fire them, which for safety reasons would be stored in a different part of the ship to the guns. Young boys, usually orphans, who were small enough to slip through tight spaces, carried this powder along tiny passages and galleys.
Because of their agility the lads became known as ‘powder monkeys’ and by association the brass trays used to hold the cannonballs became known as the brass monkeys. These trays had sixteen cannonball sized indentations which would form the base of a cannonball pyramid. Brass was used because the balls would not stick to or rust on a brass plate as they did to iron trays, but one drawback was that brass contracts much faster in cold weather than iron. This meant on severely cold days the indentations holding the lower level of cannonballs would contract, spilling the pyramid over deck, hence the saying ‘cold enough to freeze the balls off a brass monkey’. I didn’t think it had anything to do with our little jungle friends.
Extract from Money for Old Rope Parts 1 & 2