" The phrase “cold enough to freeze the balls off a brass monkey” is often said to refer to a metallic grid with circular holes in it, set under a pyramid of cannonballs on a ship’s deck to keep it stable. When this “brass monkey” got cold enough, the metal contracted and the cannonballs all popped out. In fact, the phrase means exactly what it says; the fake nautical euphemism is an attempt to make its rude humor more acceptable. "

― John Lloyd , The Second Book of General Ignorance: Everything You Think You Know Is (Still) Wrong