What is an adjective for genuinely thinking oneself as skillful or knowing a lot about a particular subject but is actually inexperienced?
Jack thought knew a lot in this area, but he actually didn't. Jack was ____.
This is different from the question What is a word or phrase for someone who professes to know something but actually doesn't? : This question is asking about a deliberately false claim, but I am asking about a genuine claim.
EDIT: I'm not looking for a slang expression. I updated the example. I am looking for a formal word.
It has some meaning overlapped with "overconfident."