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
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 (one) is 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
References in classic literature
"You don't know what you're talking about," continued the General, seating himself upon a large cut diamond.
The Emerald City of Oz
It would take an omniscient Deity to know what you're talking about."
The Age of Innocence
'You don't know what you're talking about!' cried Humpty Dumpty.
Through the Looking Glass
"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."
Don Quixote
You don't know what you're talking about. That's where the dog belongs, and that's where he'll be happiest."
Love of Life And Other Stories
"I don't know what you're talking about, I'm afraid," said the younger man, rather suspiciously.
The Man Who Knew Too Much
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.
'No way, Jose,' says 'Locsin in nixing calls to tap int'l help vs China aggression
He said the woman asked, "Where is it?" and he replied: "I don't know what you're talking about."
Footballer 'scared' in theft bid
He said the woman asked "where is it?" and he replied: "I don't know what you're talking about."
YOUNG DON 'THOUGHT HE WOULD BE KILLED' Trial hears man 'smashed car window with knife' Young Don says he feared for his life
JURGEN KLOPP has told TV pundit Gary Neville: 'You don't know what you're talking about'.
RUBBISH; Klopp tells Nev to shut up about Kop potential and that 'it's easy just to talk in an office'; SHUT IT, NEVILLE
"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."
Star maker Simon Cowell gets his place on Hollywood Walk of Fame
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.
Goalkeepers are the victims of Fifa - Talking balls at Russia 2018
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.
SHAPE SHIFTER
"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.
Mark Hamill Disagrees With Rian Johnson's Screenplay For Luke Walker
Asked away from court about the disqualification, Iqbal replied: "I don't know what you're talking about."
Millionaire 'lord' gets driving ban over Roller