Let the cat out of the bag
Definition 1 of 1
Pro Tip 1/3Accidental Disclosure
Used when someone reveals a secret without intending to do so. It suggests a mistake, not a deliberate action. 
I didn't mean to, but I let the cat out of the bag about her surprise party.
Pro Tip 2/3Figurative Phrase
An expression not about cats or bags. Understand it as a colorful way of saying 'reveal a secret'. 
Once Mike let the cat out of the bag, everyone knew about the layoffs coming.
Pro Tip 3/3Informal Context
Best used in casual conversations. Might be too informal for serious or formal writing. 
At the family reunion, Jenna let the cat out of the bag about her engagement.