The local radio station used this weather phrase the other day. It means it’s going to be cold. You got that right? Brass monkeys = frigid weather. Neither did we so we went to the expert on this matter, the expert being the Internet and our “facts” from Michael Quinion. He writes about international English from a British viewpoint. He also sounds like a pompous ass. Here’s his answer:
The full expansion of the phrase is cold enough to freeze the balls off a brass monkey and is common throughout the English-speaking world, though much better known now in Australia and New Zealand than elsewhere. This is perhaps surprising, since we know it was first recorded in the USA, in the 1850s. It is often reduced to the elliptical form that you give (perhaps in deference to polite society — for the same reason, it has been modified in the US into freeze the tail off a brass monkey).
He then precedes into a tirade about Napoleon, cannons, and the mid-19th century. But really, he had us at “balls”.