The full sentence is this: "Anyway, it was December and all, and it was cold as a witch's teat, especially on top of that stupid hill." The expression 'as cold as a witch's teat' is a figure of speech, or simile, meaning that it was very cold indeed. It isn't used in everyday English.

It is widely used even by people who haven't read "Catcher in the Rye", but I wouldn't use it in polite company. Essentially, because some people take "teat" as a vulgar word. Many people don't understand this idiom as it relates to the witch trials in the 1600's.