COLD ENOUGH TO FREEZE THE BALLS OFF A BRASS MONKEY

A wee bit of history:

CANNON BALLS

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 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, folks thought that was just a vulgar expression.