let the cat out of the bag

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let the cat out of the bag [idiom]

to reveal a secret unintentionally; to make a secret known without wanting to do so

let the cat out of the bag
Example: 

Don’t tell Ted about the surprise party. I’m sure he can’t keep it as a secret. He is always careless and forgets. He’ll let the cat out of the bag and Maya will know about the surprise party before.

Oxford Advanced Learner's Dictionary

let the ˈcat out of the bag idiom

to tell a secret carelessly or by mistake

• I wanted it to be a surprise, but my sister let the cat out of the bag.

Cambridge Advanced Learner's Dictionary

let the cat out of the bag
let the cat out of the bag
to let a secret be known, usually without intending to:
I was trying to keep the party a secret, but Mel went and let the cat out of the bag.