Hey Minnflyer this is cold.

It was necessary to keep a good supply of cannon balls near the cannon on
old war ships. But how to prevent them from rolling about the deck was the
problem.
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, for reasons unknown, a
Monkey. But if this plate were made of iron, the iron balls would
quickly rust to it. The solution to the rusting problem was to make
them of brass - hence, 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 just a vulgar expression,
didn't you?