to allow a secret to 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."
Where does this expression come from?
A cartoon by Trevor Bryden on the Cambridge Dictionaries Online blog illustrates the origin of this English phrase.
Have you ever been guilty of letting the cat out of the bag?
Definition from Cambridge Advanced Learner's Dictionary and Thesaurus.