This is June, which is normally not the time to wax lyrical about metaphors for cold weather, unlike, say, February; still, having come across this explanation in the summer, I am loath to hold it back for six months, so here we go with “cold enough to freeze the balls off a brass monkey”:
This term has nothing to do with testicles or primates, and a good deal of debate remains to this day regarding the origin of the phrase. In the days of smoothbore cannon, particularly ashore, ready-use cannon balls were stored near the guns. The balls were stacked in a “monkey,” a metal frame which was laid on the deck to help contain the bottom layer of the pyramid of cannon balls. Monkeys were typically made of brass (though monkeys made of rope were used as well). In extremely cold temperatures, the brass monkey shrank more than the iron cannon balls, and the stack of balls would collapse or perhaps ice which formed under the balls pushed them up enough to break them loose. The root of the debate is whether such an event is possible at all, though the phrase appears to be more a traditional exaggeration than an engineering possibility.
My late brother, a seafaring man early in his all-too-short life, likely would have opined that even the most egregious exaggerations had some basis in truth. And God knows I’ve seen enough cannonballs stacked in pyramids.