
let the cat out of the bag
/ˌlet ðə ˌkæt aʊt əv ðə ˈbæɡ/
to reveal a secret accidentally
let the cat out of the bag in a sentence
“She let the cat out of the bag about the surprise party.”
Origin of let the cat out of the bag
Possibly from fraudulent market practice of substituting cat for piglet in bag