let 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
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    Antonyms:
    keep one's mouth shut, keep quiet, shut one's mouth
    refrain from divulging sensitive information; keep quiet about confidential 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
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