Word for someone talking about something as if he understands it when he doesn't

Not one word but there is an idiomatic phrase:

talk through one's hat

1. (idiomatic) To speak lacking expertise, authority, or knowledge; to invent or fabricate facts.  

2. (idiomatic) To assert something as true or valid; to bluff.

Thus, bluffing somewhat has this sense also.

There is another similar idiom but it has other connotations as well:

whistle in the dark: (idiomatic, US) To speak of something despite having little knowledge of it.

A know-it-all is a person who acts as though he or she had better knowledge or understanding than anyone else. It is a hard thing to admit to being, but it is a term for someone who pretends to knowledge he doesn't have.

Once, my husband and I heard someone apologize for being a know-it-all by saying, and now I've said far more than I actually know about the subject. We thought it a humorous way to admit to the fault, and it became a family joke.