let the cat out of the bag

/L EH1 T DH AH0 K AE0 T AW0 T AH0 V DH AH0 B AE0 G/
verb
  1. 1

    To disclose a secret; to let a secret be known, often inadvertently.

    โ€œIt was going to be a surprise party until someone let the cat out of the bag.โ€

Source: https://en.wiktionary.org/wiki/let%20the%20cat%20out%20of%20the%20bag