they are colder than a witches' ta-ta...
possible origin : In the age of sail, on the cannon deck of a warship, next to each cannon, cannonballs were piled up at the ready, in a pyramid, which the sailors called a witches tit. It was built on a brass plate with holes cut in to hold the cannonballs from rolling off as the ship listed, rolled and pitched. The sailors called the plate a brass bra. Because brass expands and contracts with temperature at a greater rate than iron, rare extreme cold on the cannon deck would cause the brass plate to shrink to the point that the cannonballs would be released from the pyramid and roll over the deck. “Colder than a witche’s tit in a brass bra” is a saying that evolved from that phenomenon.