Loose lips sink ships
Unguarded talk may give useful information to the enemy.
This phrase was coined as a slogan during WWII as part of the US Office of War Information's attempt to limit the possibility of people inadvertently giving useful information to enemy spies. The slogan was actually 'Loose Lips Might Sink Ships. This was one of several similar slogans which all came under the campaigns basic message - 'Careless Talk Costs Lives'.
The slogan was in use by 1942, as this example from the Maryland paper The News, May 1942 shows:
At countians [attendees at the local county school] registered in the high school lobby before the opening of the meeting, they were surrounded on all sides by placards bearing such admonitions as "Loose Lips Might Sink Ships", "Defense On The Sea Begins On The Shore", "Defense In The Field Begins In The Factory" and patriotic creeds and slogans.