It is shortened from The King of Rock'n'Roll.
Supposedly, this title was first bestowed on Elvis in the April 19, 1956 Waco News Tribune newspaper.
Variety also called him this in November of that year, and this is the source that gets the credit for it to this day.
He hated being called The King, although he didn't mind King of Rock'n'Roll.
Personally, I've always thought both to be stupid monikers.