Of Cannonballs and Monkeys
In the heyday of sailing ships, all war ships and many freighters carried iron cannons. Those cannon fired round iron cannon balls. It was necessary to keep a good supply near the cannon, but prevent them from rolling about the deck. The best storage method devised was a square- based pyramid with one ball on top, resting on four resting on nine which rested on sixteen.
Thus, a supply of thirty 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 or rolling from under the others. The solution was a metal plate called a "Monkey" with sixteen round indentations. If this plate was made of iron, the iron balls would quickly rust to it. The solution to the rusting problem was to make "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 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 have had dirty thoughts, haven't you?}

I just had to share this with my fellows!
Lustman

This one's not really all that obscure, but most people have never understood the expression that something was going "balls to the wall," meaning fast, or flat out.

The expression came from the old steam engines of the nineteenth century that used a speed govenor to prevent them from over-reving and destroying themselves.  The mechanism used 4 balls (usually brass), that were mounted vertically on articulated arms and spun around (on a vertical axis).  

As the engine went faster and faster, the centrifugal force of the balls flung them out further and further, until they would finally trip a mechanism to slow the engine back down. Hence, when the "balls were to the wall" that was the ultimate, as fast as you could go before disaster.  Or in 21st century lingo "radical" or "extreme".  If you've ever seen a steam engine explode, it's "awesome dude."

Amusing anecdote, but apocryphal as well.

See http://www.snopes.com/language/stories/brass.htm