A: You look upset. What happened?

B: I have been planning a surprise party for my father for weeks. But yesterday my sister accidentally let the cat out of the bag about the party, ruining the plan.”

Note: The origin comes from olden times when unscrupulous merchants would sometimes dishonestly sell cats in bags instead of the pigs they advertised, hoping the buyer wouldn’t notice until it was too late.

In modern times, it means accidentally exposing a hidden truth or secret, often causing surprise or trouble.