My source was raised on a farm in Nebraska, and during the winter, snowstorms can be frequent and it can get very cold.  He remembered his father coming inside from running an errand.  After removing his outermost coat and hanging up his hat, my source’s father grinned and used this folk metaphor to describe the temperature.  My source found the comment amusing.  He laughed and then left to find his dad a blanket to warm up with.

Many times, my source has described many hardships of being raised on a farm, mainly the chores.  At one point, it was his responsibility to milk, feed, brush, and take care of the cows.  A cow’s utter is not protected by hair, and under winter conditions can become very cold.  It is important that the utter does not get too cold because the cow’s tits will actually become chapped, and the cow will become sick and stop producing milk.  To prevent this, my source would be sent out and would have to rub a protective balm on each cow’s tits.  My source distinctly remembers how cold they felt.  Now, my source made sure to explain this to me, because if he knew a cow’s tits were freezing, it made him wonder how much colder a witch’s tit would be.  And for it to be colder than a witch’s tit, then it must be seriously cold outside.

My source thinks that the metaphor was developed through similar reasoning.  Also, he mentioned that this metaphor is mostly shared by people who live in colder climates, because he has rarely heard it since he moved to California over thirty years ago.  In my opinion, I believe the metaphor was likely developed by a drunk who stepped out into the cold and decided to exclaim the temperature was colder than two completely unrelated words.  Their friends must have found this hilarious and the metaphor would have caught on and spread by word of mouth.