Colder than a whatnow?

Seeing how its Halloween, and its -5 Celcius outside, its a good time to:

Give the history behind the phrase "colder than a witch's tit" (one of my father's personal favourites)

Also see the Colder Than a Witch's Tit cartoon collection and a discussion of the British variant: Colder than a witch's teat in a brass brassiere.