The expression "mum's the word" is used to say that you have promised not to tell anyone someone's secret.


For example, if you are passing a secret along, you can use this expression:
A: We're having a surprise birthday party for Tyler on Saturday.
B: Awesome.
A: Mum's the word, okay?
B: Got it.


The "mum" in this phrase isn't mother. It's a Middle English word meaning "silent".
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