....but why?

The origin and true meaning of this phrase is actually totally unrelated to any form of primate or its testicles.
Back in time to the period of the Napoleonic War, the great gunships of this time carried many cannons on various gun decks.
As an efficient method of storage and delivery of cannon balls to the cannon for firing, a "Monkey" (this term is used to define a table and/or a rail) made of brass was used to hold the balls.
In very cold temperatures the brass would contract or even break thus allowing the cannon balls to roll off the Monkey onto the gun deck.
Hence the sailors would say "it is cold enough to freeze the balls off a Brass Monkey".

.....either way; brazing rods would be required!

Omega display showed -7.5C at 7am this morning

unlucky alf wrote on 19. Dec 2009 at 12:30:

oh i bet that hurt 

Some suffer more than others

unlucky alf wrote on 19. Dec 2009 at 12:30:

I found an ice cube in my pants after walking back from the paper shop. I put it on the radiator and a couple of minutes later it went PAAARP