Mum's the word

To keep quiet about something.

The 'mum' in this phrase isn't mother. The 'mum' of 'mum's the word' is 'mmm' - the humming sound made with a closed mouth, indicating an unwillingness or inability to speak.



Ex: “Don’t tell Jenny what I said. Understand? Mum’s the word.”

Spanish: callarse la boca



German: Nichts verraten!