Used to describe something that is insanely cold.

Back in the day, witches were believed to be evil, i.e. "cold of heart." A witches left tit (or teat) is close to the heart, therefore a witches left tit is very cold.

Also used to describe an unusually mean or nasty person.
"Stop being cheap and turn up the thermostat, it's colder than a witches left tit in here."

Canadian toilet seats in the wintertime.

"Who doesn't love a basket of puppies dressed up like the cast of Harry Potter?! That person is as cold as a witches left tit."
by allaspiaggia January 16, 2013
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