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
References in classic literature ?
"You don't know what you're talking about," continued the General, seating himself upon a large cut diamond.
It would take an omniscient Deity to know what you're talking about."
'You don't know what you're talking about!' cried Humpty Dumpty.
"Hush, girl," said Teresa; "you don't know what you're talking about; the gentleman is quite right, for 'as the time so the behaviour;' when it was Sancho it was 'Sancha;' when it is governor it's 'senora;' I don't know if I'm right."
You don't know what you're talking about. That's where the dog belongs, and that's where he'll be happiest."
"I don't know what you're talking about, I'm afraid," said the younger man, rather suspiciously.
Hijo, you don't know what you're talking about. We've never relied on the international community but almost exclusively on our balls and brains.
He said the woman asked, "Where is it?" and he replied: "I don't know what you're talking about."
He said the woman asked "where is it?" and he replied: "I don't know what you're talking about."
JURGEN KLOPP has told TV pundit Gary Neville: 'You don't know what you're talking about'.
"If anyone says fame is a bad thing, I don't know what you're talking about. It's the best thing in the world."
David de Gea described it as "really strange" before his performance in Spain's 3-3 draw against Portugal that would fit a similar description."I know what you're talking about.
I know what you're talking about, stopping old floorboards showing through the carpet and underlay, I think 6mm plywood would be fine, however if you're worried it won't do the trick, just up it to 9mm.
"I don't know what you're talking about," (http://www.ibtimes.com/star-wars-last-jedi-find-out-more-about-mark-hamills-secret-role-2629605) Hamill said ."I'm not telling.
Asked away from court about the disqualification, Iqbal replied: "I don't know what you're talking about."
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