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?
A dilettante is a person who's interested in a subject, but doesn't have any real in-depth knowledge about it.
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)
You could call the person a "poseur" which Merriam-Webster defines as:
: a person who pretends to be what he or she is not : an affected or insincere person
It's pejorative without being profane.
Sciolist is the most precise.
Oxford dictionary: A person who pretends to be knowledgeable and well informed.
You might be looking for the word claim:
claim, verb. state or assert that something is the case, typically without providing evidence or proof. (Google)
Also related is the word purport:
purport, verb. to claim to be or do a particular thing when this claim may not be true. (M-W)
You might also consider assert, contend, or maintain, although none of these necessarily implies that the speaker "in fact knows nothing about" the topic.
If the person knows he is misleading others and there are no consequences of the act other than feeding his vanity or social status, then he is a "poseur". If the consequences are a transfer of something of value (money, opportunity, affection) then he is a "fraud". This can even reach the level of illegality if his identity, abilities or services are misrepresented in a contract.
If he does not realize he is "out of his depth", he is merely a fool to be pitied and, if there is a way to do so tactfully, gently informed on the point in question.
Many of those posting answers have assumed that the person knows he is unqualified to speak on the subject and is trying to tease or deceive. This is quite often not the case. Every one of us at times has a plausible but mistaken belief that he is well informed on a topic.
To take striking example, when the Internet first became widely known in 1993, many people with experience in computing and business felt qualified to assess its potential. At dinner parties they offered detailed and highly persuasive arguments backed by specific facts to prove that it would never be particularly popular or useful. They knew (correctly) that the general public did not enjoy using computers. They knew that turning the internet into the promised universal medium for trade and communication would require mind-boggling amounts of money and effort and the full cooperation of numerous set-in-their-ways corporations.
But they were wholly uninformed about the only fact that was relevant. They had not caught the bug so did not know just how compelling the World Wide Web was. They did not know it had the power to convert a hostile public into avid computer users. They did not hear the rising din of millions of keyboards. They did not know the Internet was a juggernaught before which all of their well-reasoned objections were as naught.
They were not liars, frauds, quacks, impostors, phonies, or shams. They were misinformed or ill-informed.
In a lecture room, bluffing;
in a pub, blagging;
when it's to boost yourself, it's bragging.
In the electronic age it's blogging.
Three of these words occur in this blogspot entry: http://perpetual-lab.blogspot.co.uk/2006/06/blog-blag-brag_08.html
I think the most apt term here is "humbug":
Definition from dictionary.com
something intended to delude or deceive.
the quality of falseness or deception.
a person who is not what he or she claims or pretends to be; impostor.
something devoid of sense or meaning; nonsense: a humbug of technical jargon.
British. a variety of hard mint candy.
verb (used with object), humbugged, humbugging.
to impose upon by humbug or false pretense; delude; deceive. verb (used without object), humbugged, humbugging.
to practice humbug.
- (used as an expletive to express rejection of something as being completely untrue or nonsensical.)
Note particularly definition 3.
Some of the options not yet suggested:
Fraudulent, deceitful, or dishonest: a phony expert in investing.
someone who deceives people by pretending to be a particular kind of person, to have a particular skill, etc.
one who makes false claims of identity or expertise
A quack or charlatan (archaic).