It's colder than a witch's tit.

Basic meaning: It is extremely cold.
Obviously the person who thought up this idiom must have had some experience with witches or their bosoms. It would be interesting to know the source of an idiom I've heard at least several times each winter since childhood.

"It's colder than a witch's tit in a brass bra", is the full expression. It means 'PDC' (pretty damn/darn cold). When the temperatures are below freezing and the wind chill is a factor too. A cold metal support undergarment would make things even worse if you think about it.