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."