Keep your mouth shut!

Keep it zipped! / Keep your trap shut! / Button your lips! / Mum's the word!


Tieni la bocca chiusa!

Tieni la bocca cucita! / Tieni il becco chiuso! / Cuciti le labbra! / Stai abbottonato!


Meanings

Don't say a word about it!

(Informal)

Examples
Keep your mouth shut and mind your own business!
Tieni la bocca chiusa e fatti gli affari tuoi!

Keep it zipped and you'll get your reward
Tieni il becco chiuso e non te ne pentirai