well I'll be a monkey's uncle! the phrase is treated by the OED. (why should I be surprised?) it seems that the phrase probably evolved from the simple concept that monkeys don't like cold weather much. and the first thing to freeze off the ol' brass monkey was the tail!
b. Slang phr. cold enough to freeze the balls off a brass monkey: extremely cold. Also in similar and allusive phrases.
[1835 F. Chamier Unfortunate Man I. iv. 117 He was told to be silent, in a tone of voice which set me shaking like a monkey in frosty weather.] 1928 Amer. Speech IV. 123 Cold enough to freeze the tail off a brass monkey. 1937 Partridge Dict. Slang 528/2 Cold enough to freeze the balls off a brass monkey. 1972 Evening Telegram (St. John's, Newfoundland) 5 Aug. 3/1 Here's Smallwood still putting up a brass-monkey face right to the bitter end. 1973 Guardian 20 July 9 (headline) Brass monkey weather. Ibid., You ought to buy yourself a brass monkey and then you will know what a freeze is.
