My lips are sealed
Definition 1 of 1
Pro Tip 1/3Trust Assured
Use it to reassure someone that you are trustworthy and will not disclose confidential information. 
Don't worry about me spilling the beans; my lips are sealed.
Pro Tip 2/3Light-Hearted Tone
It's often used in a friendly, light-hearted manner to make the assurance of secrecy feel less serious. 
I've already heard the surprise party plan – but my lips are sealed!
Pro Tip 3/3Informal Expression
This phrase is casual, more common in informal conversations than in formal or official settings. 
Between you and me, he's planning to propose, but my lips are sealed.