Used to describe cold weather, this saying derives from the traditional Navy phrase "cold enough to freeze the balls off a brass monkey". This references the brass structure or 'monkey' where cannon balls were stacked.

"Cold enough to freeze the balls off (or on) a brass monkey" is a colloquial expression used by some English speakers to describe extremely cold weather. The Brass Monkey of Stanthorpe, Queensland, a place known for its "brass monkey weather", complete with a set of balls.

In the days when gulf wars had more to do with sailing ships than TV coverage, cannon balls were stacked on the decks in pyramids. The pyramids were held in place by a brass frame around the base, called a "monkey". When it got cold, the brass monkey contracted, and the cannon balls rolled off around the ship's deck.

The expression: "It is cold enough to freeze the balls of a brass monkey" comes from the practice of putting iron cannon balls on a dimpled brass plate on the deck of a war-ship. When very cold the brass contracted sufficiently to cause the iron balls to fall out.