Origins: Cold enough to freeze the balls on a brass monkey

The commonly held belief that the above phrase originated from a brass cannonball holder, known as a “monkey”, freezing and contracting and thereby causing the stacked cannonballs to fall, is inaccurate. This is because there is no known usage of the term monkey in that context, cannonballs were not stored in that manner and there is a long history of literary references going back to the 1840’s to various parts of brass monkeys being affected by heat and cold eg “singe the hair of a brass monkey”, “scald the throat of a brass monkey,” “hot enough to melt the nose of a brass monkey,” “cold enough to freeze the brass ears on a tin monkey.” Whilst there are oblique references to the balls of a brass monkey before World War II, the first specific recorded instances are after WW2. It is clear that the term has developed from the earlier less risqué usages and that the term means what it says.

Sources:
http://www.phrases.org.uk/meanings
http://www.snopes.com/business/names/wd40.asp