Cold Enough to Freeze the Balls off a Brass Monkey

In the pirate days, the ships were all equipped with cannons. The cannon balls were places upon a holder that were called ‘Brass Monkeys’ Since the metal used to make the cannon balls were extremely different from the metal used to make the brass monkeys, on an extremely cold day, they would contract at different rates (Different metals react differently to temperature). The cannon balls would literally fall off the holder when the temperature drop to the extremes. Hence the term.