Don't say a word about this!
(informal)

If the press gets ahold of this story we are dead meat, so: mum's the word!

Se la stampa viene a conoscenza di questa vicenda siamo fritti, perciò: acqua in bocca tutti quanti!

I finally convinced Eleonor to accept my invitation to the restaurant for her birthday, so mum's the word about the surprise party!

Finalmente ho convinto Eleonora ad accettare il mio invito al ristorante per il suo compleanno, perciò acqua in bocca sul party a sorpresa, mi raccomando!

Mum is the sound made when the lips are closed, that's why mum has meant silence since the 14th century. Shakespeare used this expression in Henry VI (1:2): Seal up your lips and give no word but mum