From Longman Dictionary of Contemporary Englishlet the cat out of the baglet the cat out of the bagTELL A SECRETto tell someone a secret, especially without intending to → cat
Examples from the Corpus
let the cat out of the bag• I'm sorry. Jim knows about last week's party. I'm afraid I let the cat out of the bag.• Inadvertently perhaps, the BiE report lets the cat out of the bag.• Some idiot's let the cat out of the bag -- Mrs Simpson realizes there's something going on. 