someone's lips are sealed
(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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
someone's lips are sealeda person is obliged to keep a secret.
Farlex Partner Idioms Dictionary © Farlex 2017