To keep quiet; to keep a secret; to not tell.
one's lips are sealed
One will not tell anyone else; one is keeping a secret.
keep quiet
To refrain from talking about something; to keep a secret.
keep shtum
Not tell anyone; especially, keep silent about something that may be s…
keep a low profile
To be discreet and quiet.
keep a lid on
To keep something secret.
button one's lip
To remain silent, especially in order to keep a secret or to avoid say…