let the cat out of the bag, to

let the cat out of the bag

To reveal a secret. How did Mom find out we were planning a surprise party for her? Who let the cat out of the bag?
See also: bag, cat, let, of, out
Farlex Dictionary of Idioms.

let the cat out of the bag

Fig. to reveal a secret or a surprise by accident. When Bill glanced at the door, he let the cat out of the bag. We knew then that he was expecting someone to arrive. It's a secret. Try not to let the cat out of the bag.
See also: bag, cat, let, of, out
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

let the cat out of the bag

Give away a secret, as in Mom let the cat out of the bag and told us Karen was engaged. This expression alludes to the dishonest practice of a merchant substituting a worthless cat for a valuable pig, which is discovered only when the buyer gets home and opens the bag. [Mid-1700s] Also see pig in a poke.
See also: bag, cat, let, of, out
The American Heritage® Dictionary of Idioms by Christine Ammer.

let the cat out of the bag

If you let the cat out of the bag, you reveal something secret or private, often without meaning to. `The Mosses didn't tell the cops my name, did they?' `Of course not,' she said. `They wouldn't want to let the cat out of the bag.' She'd known she was taking a risk in letting the cat out of the bag about Jacobs. Note: You can say that the cat is out of the bag when a secret has been revealed. The cat was well and truly out of the bag. The biggest scandal for years was about to overtake the government. Note: This expression may have its origin in an old trick where one person pretended to sell a piglet in a bag to another, although the bag really contained a cat. If the cat was let out of the bag, then the trick would be exposed.
See also: bag, cat, let, of, out
Collins COBUILD Idioms Dictionary, 3rd ed.

let the cat out of the bag

reveal a secret, especially carelessly or by mistake.
A similar metaphorical use of bag may be found in the French phrase vider le sac , literally ‘empty the bag’, meaning ‘tell the whole story’.
1996 Bernard Connolly The Rotten Heart of Europe Tim Renton …at odds with his leader on Europe, let the cat out of the bag when he told a television audience, ‘we need a strong Europe to maintain our independence from the United States and the Pacific Rim’.
See also: bag, cat, let, of, out
Farlex Partner Idioms Dictionary

let the ˈcat out of the bag

(informal) make known a secret, usually without realizing what you are doing: ‘Who let the cat out of the bag?’ ‘I’m afraid I did. I thought everybody already knew.’ Nobody knew she had been offered the job until her husband let the cat out of the bag. OPPOSITE: keep something under your hat
See also: bag, cat, let, of, out
Farlex Partner Idioms Dictionary

let the cat out of the bag

To let a secret be known.
See also: bag, cat, let, of, out
American Heritage® Dictionary of the English Language, Fifth Edition.

let the cat out of the bag, to

To give away a secret. This expression dates from an ancient practice of substituting a worthless cat for a valuable suckling pig by a dishonest tradesman in a farmer’s market. When the hapless buyer got home and opened the bag, the cat was revealed. See also pig in a poke.
See also: cat, let, of, out
The Dictionary of Clichés by Christine Ammer
See also:
  • let the cat out of the bag
  • bag that!
  • What’s in the bag!
  • What's in the bag?
  • squash in
  • squash into
  • squash into (something)
  • parcel