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.
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."