keep mum
To remain silent; to not say anything (e.g., about a secret). Make sure you kids keep mum in there or you're both grounded! You have to keep mum about Dave's retirement party—we want it to be a surprise.
keep mum
remain silent about something; not reveal a secret. informalkeep ˈmum
(informal) say nothing about a secret; stay silent: I just kept mum when she asked me where Ben was. She’d be furious if she knew.Please will everyone keep mum about Saturday. We want to give them a real surprise. OPPOSITE: blow the gaff (on somebody/something)