To let the cat out of the bag

What does the « Let the cat out of the bag » idiom mean?

According to Cambridge Dictionary, when we let the cat out of the bag, we allow a secret to be known by mistake. We reveal a secret carelessly by mistake, without intending to.

It means we tell someone something private we shouldn’t, but it usually marks the beginning of a series of issues.

Where does this idiom come from?

According to Mental Floss, this expression was first used in 1760 in a book review of The London Magazine:

« We could have wished that the author had not let the cat out of the bag. »

But if we wanted to find out where it comes from, we’d go back in time, to the Middle Age. Then, livestock included mostly piglets, chickens and ducks that were bagged for sale. But it turned out that sometimes, fraudsters were putting cats into bags instead of the actual animals advertised for sale.

Being about the same size and weight, cats were seen as cheap. What’s next makes sense for this idiom, as when unsuspecting customers returned home and discovered the secret, they were said to have “let the cat out of the bag.”

Source

You can also say: to spill the beans, to blow the gaff.

Translation into French: Vendre la mèche (To sell a lock of hair), Découvrir le pot aux roses (Discover the roses pot)