let the cat out of the bag

US /lɛt ðə kæt aʊt əv ðə bæɡ/
UK /lɛt ðə kæt aʊt əv ðə bæɡ/
"let the cat out of the bag" picture
1.

to reveal a secret, often accidentally

:
We were planning a surprise party for him, but his sister let the cat out of the bag.
I accidentally let the cat out of the bag about their engagement.