(uncountable) A child's game
in which one player stands on top of a hill or other location atop an incline, and attempts to repel
other players whose goal is to capture his position.
He twisted his ankle while playing king of the hill and was not allowed to play again.
(idiomatic, by extension) A person who has achieved a measure of success
and is considered to be a leader
in his field.
He was considered a renegade in journalism until he won the Pulitzer Prize, but now he is the king of the hill.