Summary of the eRumor
This piece of alleged history explains that in the olden days of sailing ships, cannon balls were stacked on the decks on brass plates called "monkeys." The plates had indentions in them that held the balls on the bottoms of the stacks. Brass, however, expands and contracts with the temperature and if it got cold enough, the cannon balls could fall...giving real foundation to the phrase "cold enough to freeze the balls off a brass monkey!"
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. 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.