A reference to extreme cold. According to the United States Navy Historical Center, this is a legend of the sea without historical justification. The center has researched this because of the questions it gets and says the term “brass monkey” and a vulgar reference to the effect of cold on the monkey’s extremities, appears to have originated in the book “Before the Mast” by C.A. Abbey published in 1857. It was said that it was so cold that it would “freeze the tail off a brass monkey.”. The Navy says there is no evidence that the phrase had anything to do with ships or ships with cannon balls.
The first taxi man George encountered in Brooklyn said, "It is cold fit to freeze the balls off a brass monkey."