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 2/3
Light-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 3/3
Informal 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.