let the cat out of the bag
let the cat out of the bag {v. phr.}, {informal}
To tell about something that is supposed to be a secret.
We wanted to surprise Mary with a birthday gift, but Allen let the cat out of the bag by asking her what she would like.
- Sometimes used in another form.
Well, the cat is out of the bag — everybody knows about their marriage.
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Source: A Dictionary of American Idioms