1mum 
/ˈmʌm/ adjective
/ˈmʌm/ adjective 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]
▪ 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]
mum's the word
informal — used to say that some information is being kept secret or should be kept secret ▪ We want her birthday party to be a surprise, so mum's the word. [=don't tell her about the party]
