Wiktionary
n. 1 (context idiomatic colloquial often humorous English) A person who is important or high-ranking. 2 An extremely pompous person.
Wikipedia
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 ... Lord Mayor" and "Lord High Everything Else". The name has come to be used as a mocking title for someone self-important or locally high-ranking and who either exhibits an inflated self-regard or has limited authority while taking impressive titles.
Usage examples of "grand poobah".
Recovering nicely, and soon to be put to work, by orders of his Grand Poobah-dom, Commander Osborn.
Anyway, you may be the grand poobah around here, but that doesn't mean you can tell me what to do in my private life.