Know what they are talking about - Idioms by The Free Dictionary
know what (one) is talking about (redirected from know what they are 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!
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.