Wiktionary
philosopher king
n. A very wise ruler.
Wikipedia
Philosopher king
Philosopher kings are the rulers of Plato's utopian city of Kallipolis. If his ideal city-state is to ever come into being, "philosophers [must] become kings…or those now called kings [must]…genuinely and adequately philosophize" ( The Republic, 5.473d).
Usage examples of "philosopher king".
It is not a sickness, it is what you are, and the scent of your blood is the same as what I smelled when I met Labad, the human's Philosopher King.
Every academic worth his salt wants to be a Philosopher King, but that takes a lot of money.