Wordorigins.org

This phrase is often said to have a nautical origin involving cannon balls, but cold enough to freeze the balls off a brass monkey is instead both literal and anatomical in origin. And although now the phrase is used almost exclusively in the canonical version given here, in early usage the parts of the monkey’s anatomy varied, as did the temperature, with the phrase being used to refer to hot weather as well.

The OED2’s entry on monkey includes the following citation from Frederick Chamier’s 1835 Unfortunate Man. While not in the form we’re familiar with today, it establishes monkeys as metaphorical instruments of weather measurement:

He was told to be silent, in a tone of voice which set me shaking like a monkey in frosty weather.

The earliest known use that parallels the modern form is from Herman Melville’s 1847 novel Omoo:

It was ‘ot enough to melt the nose h’off a brass monkey.

1857 sees the temperature reversed in C.A. Abbey’s Before The Mast:

It would freeze the tail off of a brass monkey.

These are followed by a string of variations from the American Civil War era and later. In Cooke’s Wearing Gray (1865):

His measure of cold was, “Cold enough to freeze the brass ears on a tin monkey.”

Other anatomical parts frozen or melted off the hapless brass monkeys have been ears, hair, and whiskers. The poor creatures have also had their throats scalded out, pants scared off, and guts rotted out. One account has the monkey’s leg talked off and one writer was hungry enough to eat the brass monkey’s balls; one hopes he was speaking metaphorically.

The balls version is first recorded in 1937 in Partridge’s Dictionary of Slang and Unconventional English. Although it is likely that many of the earlier citations were bowdlerized and originally referred to testicles as well.

The oft-cited nautical origin would have the monkey be a brass rack used to store cannonballs on board ship. According to the tale, in cold weather the rack would shrink, spilling the balls onto the deck. As we have seen, the lexical evidence doesn’t support this. While some of the earliest citations (Melville, Abbey) are in nautical contexts, they also refer to heat and other body parts. Furthermore, no one has ever produced a usage of monkey to mean a rack holding cannonballs (or anything similar). (Although there are some 17th century uses of monkey to mean a type of cannon and monkey tail is a 19th century name for a handspike used to aim and level cannon.)

And the story ignores some basic facts of physics and naval life. First, while brass, like any metal, does contract in cold weather, the amount of shrinkage is so infinitesimal that it would not cause the spilling of a rack’s contents. Also, naval ships did not store cannonballs in vertical racks; the rolling of the ship made this impractical. Instead, they were stored in holes drilled in horizontal wooden planks known as shot garlands.

(Sources: Oxford English Dictionary, 2nd Edition; Historical Dictionary of American Slang)