“Brass Monkey Weather” is an idiom describing very cold weather and derives from the saying “It’s cold enough to freeze the balls off a brass monkey.”
However, I wonder if anyone knows the origin (and meaning) of this saying?
It's actually from the Royal Navy tradition of having a pyramid of cannon balls at the top of the gang plank as you come on board a Ships of the Line. The black cannon balls were made of iron, but the triangle they were built upon (called a monkey) was made of brass. As iron and brass contract at different rates, when the weather was very cold, the balls would fall off the brass monkey, hence the saying “It’s cold enough to freeze the balls off a brass monkey" or "Brass Monkey Weather."
Another Royal Navy term which also found its way into civilian ships too:
The “Heads” refers to a toilet on board a ship.
In the olden days, when ships were made of wood (and men were made of steel), there were no toilets on board. If you needed a No 1 you just pointed it over the side and let go. The Skipper, however, did it from the bow and the first mate would call “Heads!” whereupon all lower ranks would avert their gaze, hold their hands over their faces and avoid the spray!