She told him to 'keep/stay mum' about the project. [=she told him not to tell anyone about the project]
We want her birthday party to be a surprise, so 'mum's the word'. [=don't tell her about the party]
I'd like to know how much they paid, but they've been 'mum' on that subject.
(..) - Same as the day before. - Except you went exploring. - I like exploring. - FATHER: Where did you go? I wanted to look in the back garden, but Mum wouldn't let me. The back garden? Well, God forbid. I expect you'll be missing those friends of yours. I even miss school. (..)2008 The Boy in the Striped Pajamas
(..) (DOG BARKING) It's me home. - Bye, Bruno! - Bye, Bruno! - MOTHER: Hello, sweetheart. - Mum, what's going on? - We're celebrating. - BRUNO: Celebrating? Your father's been given a promotion. - That means a better job. - I know what promotion is. (..)2008 The Boy in the Striped Pajamas