don't let the cat out of the bag Share definition Flag An idiom for do not tell a secret. Uses figurative language. "Don't let the cat out of the bag or the party will be ruined!" don't let the cat out of the bag by JefeOfMasers April 18, 2017 0 0 Get merch Get the don't let the cat out of the bag mug. Buy Now — $32.95 See more merch