know what (one) is 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
Farlex Dictionary of Idioms.

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
Farlex Partner Idioms Dictionary
See also:
  • know what you're talking about
  • know what (one) is doing
  • write the book on
  • wrote the book on
  • wrote the book on (something)
  • no stranger to (something)
  • know a trick or two
  • know a thing or two
  • know what (one) is about
  • know what you're doing/about