know what you're talking about

know what (one) is talking about

To be adequately knowledgeable about or experienced in something one is discussing. Mister, I don't think you know what you're talking about. There's no way these old pipes can withstand that kind of pressure!
See also: know, talk, what

know what you’re ˈtalking about

(informal) have good knowledge of something; be an expert on something: I really enjoyed that lecture. She certainly knows what she’s talking about.That history teacher doesn’t know what he’s talking about. He makes a lot of mistakes.
See also: know, talk, what