The Brass Monkey of Stanthorpe, Queensland - the place known for its "brass monkey weather", complete with a set of balls
The phrase "cold enough to freeze the balls off (or on) a brass monkey" is a colloquial expression used by some English speakers. The reference to the testes (as the term balls is commonly understood to mean) of the brass monkey appears to be a 20th-century variant on the expression, prefigured by a range of references to other body parts, especially the nose and tail.
During the 19th and 20th centuries, small monkeys cast from the alloy brass were very common tourist souvenirs from China and Japan. They usually, but not always, came in a set of three representing the Three Wise Monkeys carved in wood above the Shrine of Toshogu in Nikkō, Japan. These monkeys were often cast with all three in a single piece. In other sets they were made singly. Although three was the usual number, some sets of monkeys added a fourth, with its hand covering its genitals. Old brass monkeys of this type are collectors' items.Michael Quinion, advisor to The Oxford English Dictionary and author of World Wide Words says "It’s more than likely the term came from them".
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.
An early known recorded use of the phrase "brass monkey" appears in the humorous essay "On Enjoying Life" by Eldridge Gerry Paige (writing under the pseudonym "Dow, Jr."), published in the New York Sunday Mercury and republished in the book Short Patent Sermons by Dow, Jr. (New York, 1845):
Another early published instance of the phrase appeared in 1847, in a portion of Herman Melville's autobiographical narrative Omoo:
"It was so excessively hot in this still, brooding valley, shut out from the Trades, and only open toward the leeward side of the island, that labor in the sun was out of the question. To use a hyperbolical phrase of Shorty's, 'It was 'ot enough to melt the nose h'off a brass monkey.'"
An early recorded mentioning of the freezing a "brass monkey" dates from 1857, appearing in C.A. Abbey, Before the Mast, p. 108: "It would freeze the tail off a brass monkey".
Michael Quinion's World Wide Words website says:
"it was first recorded in the USA, in the 1850s...in the oldest example known, from Herman Melville’s Omoo (1850)"
The Story of Waitstill Baxter, by Kate Douglas Wiggin (1913) has "The little feller, now, is smart's a whip, an' could talk the tail off a brass monkey".
The Ivory Trail, by Talbot Mundy (1919) has "He has the gall of a brass monkey".
GRACE: Well, I guess I'd better be "barging along," as they say. I'm sure it's getting colder by the minute.
TOM: Yes—I think we'd best bring the brass monkeys in tonight.
The "brass monkey" is the nickname of the house flag of the Cunard Line, adopted in 1878, a lion rampant or on a fieldgules holding a globe. The reference is almost certainly irreverent humour, rather than a source of the expression, of which variants predate it.
US patent 4634021 describes "A release mechanism is disclosed for releasing an object such as a ball from a body under the force of gravity. A bimetallic element obstructs or opens an opening in the body for retaining or releasing the object depending upon the temperature of the bimetallic element. The release mechanism may be incorporated into a novelty "brass monkey" for "emasculating" the monkey when the temperature decreases to a predetermined temperature at which the balls in the "brass monkey" are permitted to drop to a base which is designed to produce an audible sound when struck by the balls."
The OED does not record the term "monkey" or "brass monkey" being used in this way.
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.
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.
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.
The physics does not stand up to scrutiny. 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.
More likely the reference is almost certainly 16th to 18th century humour, just like it is used today to emphasize how cold it is.