How cold is it?

The Etymology of favoured expressions. I don't know if
this is true or not, but it ought to be.

Many ships in the age of sail carried cannons. Cannon of
the times required round iron cannonballs. The gunmaster
wanted to store the cannonballs such that they could be
instantly ready when needed, yet not roll around the gun
deck crushing toes and breaking legs which was very bad for
morale among the deck crew.

The solution was to stack the cannon balls in a square-
based pyramid next to each cannon. The top level of the
stack had one ball, the next level down had four, the next
had nine, the next had sixteen, and so on. Four levels
would provide a stack of 30 cannonballs.

The only real problem was how to keep the bottom level from
sliding out from under the weight of the higher levels. To
do this, they devised a small brass plate (called a "brass
monkey") with one rounded indentation for each of the 16
cannonballs in the bottom layer. Brass was used because the
cannonballs wouldn't rust to the "brass monkey," whereas
they would rust to an iron one.

However, when temperature falls, brass contracts in size
faster and more than iron. As it got cold on the gun
decks, the indentations in the brass monkey would get
smaller than the iron cannonballs they were holding. If the
temperature got really cold, the bottom layer of
cannonballs would pop out of the indentations spilling the
entire pyramid over the deck. Thus it was, quite literally,
"cold enough to freeze the balls off a brass monkey."

[CyberSurfer]

As far as I know, that's true, because I learnt it in Higher History last year!