(one's) lips are sealed
One will not tell anyone; one will keep this a secret. Most often used in the expression "my lips are sealed." A: "Please don't tell anyone about this, I'm so embarrassed." B: "My lips are sealed." I told him not to say anything, and he said his lips are sealed, so hopefully he'll be able to keep a secret this time.
lips are sealed, one's
One will reveal nothing, especially about a secret. For example, You can trust me with the details of the lawsuit-my lips are sealed. [Early 1900s]