Grand Poobah is a term derived from the name of the haughty character Pooh-Bah in Gilbert and Sullivan’s The Mikado (1885). 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,
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.

Roy H. Williams is the Grand Pooh-bah of Wizard Academy.
He is also my boss.
…and I am in so… much… trouble.

Poofy Guy on a Short Leash, by James Christensen