your 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.
See also: lip, seal

your lips are sealed

If you say that your lips are sealed, you mean you will keep a secret that someone has told you. As for anything told to me in confidence, well, my lips are sealed. `The Player' is worth seeing for its deeply funny ending alone but my lips are firmly sealed on that. Note: You can also say that someone has sealed lips. Most people refused to talk to him about the murder. Everywhere he went he met sealed lips and a wall of silence.
See also: lip, seal