This was a slogan used to remind military personnel and civilians that talking about troop movements could result in enemy intelligence agents collecting data and being able to report when troop ships took off. This information was passed to submarines and aircraft that would attack them.
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"Loose Lips sinks Ships".
It means that you should be careful of unguarded talk as you may something that could lead to disaster. Another was Loose Lips Sink Ships.
Hood (battlecruiser)
the Lusitania
..... sorry __ Sink them.