What is a word or phrase for someone who professes to know something but actually doesn't?

Is there a word or phrase that describes when a person professes to know about a certain subject, but in fact knows nothing about it?

Solution 1:

A dilettante is a person who's interested in a subject, but doesn't have any real in-depth knowledge about it.

Solution 2:

The word quack often applies to those claiming medical knowledge, but it is not limited to that subject:

an unqualified person who claims medical knowledge or other skills (Collins English Dictionary)

a person who pretends, professionally or publicly, to skill, knowledge, or qualifications he or she does not possess; a charlatan. (Random House)

A charlatan meets this description as well:

A person who makes elaborate, fraudulent, and often voluble claims to skill or knowledge; a quack or fraud. (American Heritage Dictionary)

someone who professes knowledge or expertise, esp in medicine, that he or she does not have (Collins English Dictionary)