From BiblePortal Wikipedia

Webster's Dictionary [1]

(n.) The title of the supreme ruler in certain Eastern countries, especially Persia.

The Nuttall Encyclopedia [2]

"King"), an abbreviation of Shah-in-Shah ("King of Kings"), the title by which the monarchs of Persia are known; may also be used in Afghanistan and other Asiatic countries, but more generally the less assuming title of Khan is taken.