let the cat out of the bag

[let the cat out of the bag] {v. phr.}, {informal} To tell aboutsomething that is supposed to be a secret.

We wanted to surpriseMary with a birthday gift, but Allen let the cat out of the bag byasking her what she would like.

- Sometimes used in another form.

Well, the cat is out of the bag - everybody knows about theirmarriage.

Compare: GIVE AWAY(3), LET OUT(2), SPILL THE BEANS.