RichThesaurus
let the cat out of the bag
vs
open one's mouth
let the cat out of the bag
verb
To disclose a secret; to let a secret be known, often inadvertently.
open one's mouth
verb
to speak
How often have the words
let the cat out of the bag
and
open one's mouth
occurred in a corpus of books?
(source:
Google Ngram Viewer
)