The phrase "cold enough to freeze the balls off a brass monkey" is a colloquial expression used by some English speakers. The reference to the testes (as the term balls is commonly understood to mean) of the brass monkey appears to be a twentieth century variant on the expression, prefigured by a range of references to other body parts, especially the nose and tail.

These earlier expressions would seem to indicate that the brass monkey took the form of a real monkey, rather than being the name for some dissimilar object, without a nose and tail, such as a tray to hold cannonballs as has been theorised.

During the 19th and 20th centuries, small monkeys cast from the alloy brass were very common tourist souvenirs from China and Japan. They usually, but not always, came in a set of three representing the Three Wise Monkeys carved in wood above the Shrine of Toshogu in Nikkō, Japan. These monkeys were often cast with all three in a single piece. In other sets they were made singly.

The theory of the Three Wise Monkeys as being the source of the expression is supported by Michael Quinion, advisor to The Oxford English Dictionary and author of World Wide Words.

Whether or not it was these brass monkeys—common objects that could be purchased in any store selling Asian goods—or some other object, possibly not closely related to actual monkeys, has been a subject of speculation, theories and association.

The phrase, as it is currently used, is found in most English-speaking countries, and is sometimes abbreviated to "brass monkey weather". According to Quinion, the expressions relating to "brass monkeys" have more currency in Australia and New Zealand than elsewhere.