Brass monkey (colloquialism)
"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. In New Zealand, the phrase is often shortened to “brass monkey weather,” used colloquially to describe extremely cold conditions.
The reference to the testes (as the term balls is commonly understood to mean) of the brass monkey appears to be a 20th-century variant on the expression, prefigured by a range of references to other body parts, especially the nose and tail.