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.

Used to indicate that one will not disclose a piece of information.

He/I will keep this secret. Although the idea of keeping one’s mouth tightly shut is much older and sealing up someone else’s lips dates from the late 1700s, this particular expression became current only in the early twentieth century. It was much repeated by Prime Minister Stanley Baldwin when asked about the rumored abdication of King Edward VIII, who wished to (and eventually did) marry a divorced American, Wallis Simpson. See also mum's the word.