KIM MERCURIAL?
Idea.
POTUS talks in code.
Anons already looked up definition of mercurial.
What if instead of literal, it is symbolic?
Mercury, Latin Mercurius, in Roman religion, god of shopkeepers and merchants, travelers and transporters of goods, and thieves and tricksters. He is commonly identified with the Greek Hermes, the fleet-footed messenger of the gods.
Could POTUS be saying that Kim was a messenger?
That would be consistent with anons' thinking that somebody else is actually in control of North Korea.
Just throwin' it out there for anons' consideration.