What is the origin of the idiom "pig's ass" and when to use it?

The idiom "pig's ass" originates from the phrase "when pigs fly" or "pigs might fly," which is used to express extreme skepticism or disbelief about the possibility of something happening. The addition of "ass" to the idiom adds emphasis and a touch of vulgarity. The origin of the phrase is not entirely clear, but it likely emerged as a humorous and exaggerated way to dismiss something as highly unlikely or impossible.