witch's were once imagined as old hags with wrinkly skin and icy blood. ha! So the phrase "...colder than a witch's tits..." was used during very cold weather. When someone who still thinks this way came into a warm house or room, often, the phrase would come to mind.
Ol' Harry came into the cabin wrapped up in almost everything he had. The blizzard was just reaching it's peak. He said shiveringly, "DDDDamn! It's ccccolder than a witch's tits out thththere!"