Let the cat out of the bag

To share a secret that wasn't suppose to be shared.



This relates to the common fraud of substituting a cat for a piglet at markets. If you let the cat out of the bag you disclosed the trick - and avoided buying a pig in a poke (bag). This form of trickery is long alluded to in the language and 'pigs in a poke' are recorded as early as 1530



Ex: I’m afraid I may have let the cat out the bag about Dan’s surprise party.



Spanish: abrir la boca



German: die Katze aus dem Sack lassen