As far as I understand, a moon is an object in permanent orbit around a planet, dwarf-planet, asteroid, etc. If there was another object permanently orbiting this moon, would that be a moon-moon?
Is there any natural object of such kind that we know of?
If no, why are such objects so unlikely?
Edit: this is question is different from Do moons have moons? because I explicitly ask for a reason. Also, the terminology is included here.