Idiom

''Let the cat out of the bag''
It means to give away a secret.
Centuries ago in england, people used to buy pigs and other little animals from merchants, and they would give them to you in a bag.
Some merchants would give you a cat in a bag instead of the animal you selected.

Hello bob, what's up?
Yeah.. nothing.
Something happened to you, its pretty obvious, please tell me, I might be able to help, let the cat out of the bag buddy.
Thanks, my friend, the truth is I stole something from the oxxo last night.
You should give it back, theft is a serious matter.