You searched for: “aliud corde premunt”
Aliud corde premunt, aliud ore promunt. (Latin saying)
One thing they conceal in the heart, they disclose another with the mouth.

Another version is, "No use trying to keep a secret." A warning that when more than one person knows something, it is no longer a secret because there is usually someone who will talk about it.

This entry is located in the following units: ali- (page 1) cor-, cord-, cour- (page 1) Latin Proverbs, Mottoes, Phrases, and Words: Group A (page 13)