Sagan of Jerusalem
in Dryden's Absalom and Achitophel, is designed for Dr. Compton, Bishop of London; he was son of the Earl of Northampton, who fell in the royal cause at the battle of Hopton Heath. The Jewish sagan was the vicar of the sovereign pontiff. According to tradition, Moses was Aaron's sagan.
The Sagan was the vicar of the Jewish pontiff. Thus they called Moses “Aaron's Sagan.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894