“Loose lips sink ships.”
Literally: “Free-speaking lips sink ships.”
In other words: “beware of unguarded talk.”
It was an American wartime slogan used during the Second World War to urge civilians not to share information with anyone.
It is an old military motto, but it remains perfectly valid today:
in certain offices, with just a couple of casual coffee-break chats, one can cause more damage than an entire fleet.