Two of my favorite words: "ultracrepidarian" and "mumpsimus." They're closely related.
ultracrepidarian = one who pretends to greater knowledge than he or she actually possesses; esp., a know-nothing know-it-all.
mumpsimus = one who persists in error despite irrefutable correction. President. G.W. Bush was a mumpsimus when it came to saying "nuclear."
Neither Webster's New World College Dictionary nor Merriam-Webster contained either word. But Dictionary.com had 'em: ultracrepidarian