Is “pony” an acronym? Nope.
It’s the lowest priority there is. It’s the “I want a pony! No, you can’t have a pony” priority. Or as the classic image has it:
This priority is properly reserved for requirements that would be, like, REALLY KEWL but that won’t ever be implemented. Because they’re unsolved research problems, or because they would cost more than the whole company is worth.
This is a seriously useful concept. It provides a way to say, “I recognize the value of the idea, but we can’t do it no matter how much you try.”
Do use it in your own company and let us know it works out.