South end of a north facing horse.... what does this mean?

The phrase "south end of a north facing horse" is a slang term used to refer to something of very low value or quality. It is often used in a humorous way to describe something that is considered to be worthless or unimportant. For example, someone might say that a particular car is "worth about as much as the south end of a north-facing horse" to indicate that it is in very poor condition and not worth much money.