APAD: "My lips are sealed"

Meaning:
-  If you tell someone that your lips are sealed, you are promising them that you will keep a secret that they have told you.
-  I cannot or will not reveal something private

Background:
The earliest known recording is from 1782 when Fanny Burney wrote in Cecilia - "I make it quite a principle to seal my lips from the moment I perceive him."
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Maowi the cat gently woke me up at 3am this morning.  I went and cleaned his litter box and said : "I'll not tell Papa the bad business you did this morning."  He looked at me and then looked away.  I believe he trusted my words and understood that "my lips are sealed".  But he didn't know I would tell the tale here.  Oops ...