You might also see or hear "mum's the word" as a way to tell those other than yourself to keep quiet, but that's a bit archaic these days.
Wikipedia says:
Mum is a Middle English word meaning 'silent',[2] and may be derived from the mummer who acts without speaking.[3] Note the similar English word "mime" (Old English "mīma", Latin "mimus") meaning silent actor or imitator.
I guess it's a word that remained in use in the U.S. but fell out of favor in other parts of the English-speaking world.