From the phrase cold enough to freeze the balls off a brass monkey. According to the U.S. Naval Historical Center , which cites the Random House Historical Dictionary of American Slang , 'the first recorded use of the term "brass monkey" appears to dates[sic] to 1857 when it was used in an apparently vulgar context by C.A. Abbey in his book Before the Mast, where on page 108 it says "It would freeze the tail off a brass monkey."' A number of false etymologies have been suggested. For more information, see Brass monkey on Wikipedia..