In the early days of the U.S. Navy when frigates ruled the oceans, canon balls were stored on brass railings called monkeys.  This was fine in warm weather; however, when the weather grew cold, the caNON balls would slip through the monkeys and fall to the deck.  This was because iron contracted more than brass, making the canon balls smaller than the holders.  The canon balls would then roll around on the deck, causing the crew to make rope holders to stack them.  Thus, we have the saying that it is so cold that it would freeze the balls off a brass monkey.  And, now you know the rest of the story.

I wonder how many brass monkeys are losing their balls tonight.

most likely an urban (Chicago?) legend.

Obviously this guy didn't read the "Master and Commander" series. I believe the story to be true or a very close representation of the truth. We also got " and Bobs your uncle" from early British sailing slang, meaning loosely if you know someone chances are you'll get a good job, deal, place at the table, whatever. Great series.

My father was not from Chicago, and I grew up out west.  He was in the navy.

He used to always say, 'better bring your brass monkeys inside' leaving out any reference to their balls.

On more than one occasion as a young child I walked around the yard on a bitterly cold day looking for a monkey, and a brass one at that.

I never found monkeys.

For over 20 years I wondered why the brass monkeys needed to come inside but the non-brass ones could stay outside.  I wondered what exactly a brass monkey was.  I wondered why anyone would have a brass monkey.  I worried about brass monkeys stuck outside.  I wondered if I would hear about brass monkeys suffering outside.

I found out the references to the monkey's balls and the story as posted here about 10 years ago from a colleague who sailed.