What do you mean?
To let the cat out of the bag means to give away a secret,
usually without meaning to, or against
someone else's wishes.
Why does it mean this?
This saying comes from a trick played by traders at country fairs.
Pig traders sometimes tried to fool customers by giving them a sack, into
which they had secretly put a cat, which was worth less money than a pig.
If the cat got out, the trick was discovered.
Example:It was meant to be a surprise party but Harry
let the cat out of the bag by asking what time it started.