let the cat out of the bag

/lɛt ðə kæt aʊt ʌv ðə bæɡ/

Other forms: letting the cat out of the bag; lets the cat out of the bag

If you let the cat out of the bag, you reveal a secret, probably one that you promised to keep to yourself.

If you promise not to reveal your friend's secret crush but then find yourself telling everyone in your math class about it, you've let the cat out of the bag. Like so many idioms, no one is positive where this saying originated, but one good guess is that it came from the old trick of substituting a cat for a piglet at a meat market. When the cat came out of the bag, the fraud would be revealed.

Definitions of let the cat out of the bag

1. verb

   divulge confidential information or secrets
   synonyms: babble, babble out, blab, blab out, peach, sing, spill the beans, talk, tattle

   spill, talk

   reveal information

   type of:

   break, bring out, disclose, discover, divulge, expose, give away, let on, let out, reveal, unwrap

   make known to the public information that was previously known only to a few people or that was meant to be kept a secret