mum's the word
• (as a request or warning) say nothing; don't reveal a secret.
• "I know I can rely upon your discretion, mum's the word"
Similar:
say nothing,
keep quiet,
don't breathe a word,
don't tell a soul,
don't give the game away,
keep it secret,
keep it to yourself,
keep it under your hat,
play dumb,
play the daft baddie/lassie,
don't let on,
keep shtum,
