Charlatan
Etymology
From Italian "ciarlatano", from "ciarlare" (to chatter, prattle), reinforced by "cerretano," an inhabitant of Cerreto, an Italian town notorious for its wandering quacks. The word thus fuses two ideas: empty patter and the traveling seller of fake cures.
noun · /SHAR-luh-tun/
A person who falsely claims special knowledge or skill in order to deceive.
What it names
The fraud who trades specifically on faked expertise — the fake doctor, expert, or guru — not just any liar.
Example
The clinic’s founder turned out to be a charlatan whose medical degree was as invented as his miracle device.
Related idea
A charlatan fakes expertise and knows it is fake; someone in the grip of the Dunning–Kruger effect sincerely overrates their own competence — the charlatan deceives others, the incompetent deceives themselves.