"Mum's the word" means "say nothing, don't reveal a secret."
Example #1: I think I'm pregnant, but mum's the word until I know for sure.
Example #2: I'll tell you about our secret hiding place, but mum's the word; don't tell anyone about it.
Example #3: Nobody must mention this project outside the office. Mum's the word!
Example #4: The information I am going to reveal to you is highly confidential, so mum's the word.
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