Cold enough to freeze the balls off a brass monkey
It is widely believed that a brass monkey is a brass tray used in naval ships during the Napoleonic Wars, for the storage of cannonballs, piled up in a pyramid. The theory goes that the tray would contract in cold weather, causing the balls to fall off. This theory is discredited by the US. Department of the Navy and the etymologist Michael Quinion and the OED's AskOxford website for five main reasons:
1. The Oxford English Dictionary does not record the term "monkey" or "brass monkey" being used in this way.
2. The purported method of storage of cannonballs ("round shot") is simply false. Shot was not stored on deck continuously on the off-chance that the ship might go into battle. Indeed, decks were kept as clear as possible.
3. Furthermore, such a method of storage would result in shot rolling around on deck and causing a hazard in high seas. Shot was stored on the gun or spar decks, in shot racks—longitudinal wooden planks with holes bored into them, known as shot garlands in the Royal Navy, into which round shot were inserted for ready use by the gun crew.
4. Shot was not left exposed to the elements where it could rust. Such rust could lead to the ball not flying true or jamming in the barrel and exploding the gun. Indeed, gunners would attempt to remove as many imperfections as possible from the surfaces of balls.
5. The physics do not stand up to scrutiny. All of the balls would contract equally, and the contraction of both balls and plate over the range of temperatures involved would not be particularly large. The effect claimed possibly could be reproduced under laboratory conditions with objects engineered to a high precision for this purpose, but it is unlikely it would ever have occurred in real life aboard a warship.
Early references to "brass monkeys" in the 19th century have no references to balls at all, but instead variously say that it is cold enough to freeze the tail, nose, ears, and whiskers off a brass monkey; or hot enough to "scald the throat" or "singe the hair" of a brass monkey. All of these variations imply that an actual monkey is the subject of the metaphor. "Balls" being the surviving phrase