From Wikipedia:
Grand Poobah is a term derived from the name of the haughty character Pooh-Bah in Gilbert and Sullivan's The Mikado (1885).[1] In this comic opera, Pooh-Bah holds numerous exalted offices, including "First Lord of the Treasury, Lord Chief Justice, Commander-in-Chief, Lord High Admiral... Archbishop of Titipu, and Lord Mayor" and Lord High Everything Else. The name has come to be used as a mocking title for someone self-important or high-ranking and who either exhibits an inflated self-regard or who has limited authority while taking impressive titles.[2] The formal term for the practice of holding multiple offices is "dual mandate".