It's true... Honest!!

[Neil]

In the heyday of sailing ships, all war ships and many freighters carried iron cannons. Those cannons fired round iron cannon balls. It became necessary to prevent them from rolling about the deck.

The best storage method devised was to stack them as a square based pyramid, with one ball on top, resting on four, resting on nine, which rested on sixteen. Thus, a supply of 30 cannon balls could be stacked in a small area right next to the cannon.

There was only one problem - how to prevent the bottom layer from sliding/rolling from under the others. The solution was a metal plate with 16 round indentations, called a Monkey. But if this plate was made of iron, the iron balls would quickly rust to it. The solution to the rusting problem was to make Brass Monkeys.

Few landlubbers realize that brass contracts much more and much faster than iron when chilled. Consequently, when the temperature dropped too far, the brass indentations would shrink so much that the iron cannon balls would come right off the monkey.

Thus, it was quite literally, "cold enough to freeze the balls off a brass monkey"!

And all this time, you thought that was a vulgar expression, didn't you?

Hehehehehe - Neil

[Rocker]

Yeah, right.

But then, how would I know? We never get frost in this part of Queensland

[AlexS]

When I was working in the bush down in the Snowy Mts. it was quite often cold enough to freeze the walls off a bark humpy.

[arose62]

I work at Garden Island - the Navy dockyard in Sydney, and we had quite a long-running investigation over this, including querying the folks on the "Young Endeavour".

General consensus seems to be similar to below:

================
From http://www.quinion.com/words/qa/qa-bra1.htm

There is a story, often repeated, that the phrase originated in naval warfare at the time of the Napoleonic wars, if not before. It is said that the stack of cannon balls alongside each gun, arranged in a pyramid on a brass plate to save space, was called a monkey. In very cold weather, it is related, the cannon balls would shrink and balls would fall off the stack.
Though monkey was a term used in this context and era (the boys bringing charges to the guns from the magazine were known as powder monkeys and there is some evidence that a type of cannon was called a monkey in the mid seventeenth century), there is no evidence for the word being applied to a pile of cannon shot.
The explanation sounds like a story that’s been woven around a term already well known and is full of logical holes: would they pile shot into a pyramid? (hugely unsafe on a rolling and pitching deck); why a brass plate? (far too expensive, and unnecessary: they actually used wooden frames with holes in, called garlands, fixed to the sides of the ship); was the plate and pile together actually called a monkey? (no evidence, as I say); would cold weather really cause such shrinkage as to cause balls to fall off? (highly improbable, as all the balls would reduce in size equally and the differential movement between the brass plate and the iron balls would be only a fraction of a millimetre).
Fun story, though.
==================

Cheers,
Andrew