Perhaps you were thinking of one of the following:
* "Lips sink into a smile": This phrase is often used to describe a person who is happy or amused.
* "Lips seal shut": This phrase means that someone is refusing to talk or reveal information.
* "Lips are sealed": This phrase is similar to "lips seal shut" and implies secrecy or silence.
If you can provide more context about where you heard this phrase, I might be able to help you understand its meaning.