Colder than a witch's teat...A witch's tit (or witch's teat, to use the older spelling) supposedly left a marking  
that witch hunters and courts would look for on the body of an accused person. Supposedly, witches would suckle their familiars, and sometimes the Devil himself, from this "unholy" body part. To find these marks, as well as insensitive spots on the skin called devil's marks--caused by the Devil's claws or teeth--the suspects were stripped, shaven, then closely examined for any blemishes, moles, or even scars that could be labeled as diabolical. To find marks invisible to the eye, the examiner would poke the victim inch by inch with a blunt needle (called a bodkin) until they found a spot that didn't feel pain or bled. Discovery of these marks or spots--one supposes they would be considered cold since they were a sign of communion with the Devil--would be "proof" of the person's dealings with Scratch, so they would be shown in full court before the execution.  NOW THAT'S COLD