informal
: not talking about something
I'd like to know how much they paid, but they've been mum on that subject.
She told him to keep/stay mum about the project. [=she told him not to tell anyone about the project]
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“Mum.” Merriam-Webster.com Simple Definition, Merriam-Webster, https://www.merriam-webster.com/simple/mum. Accessed 20 Jun. 2026.
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