On today's show we will talk about an idiom of warning . And it is not very easy to say : Loose lips sink ships . This idiom began during World War II as a slogan used by the United States Office of War Information . It was part of a propaganda campaign . So , the slogan “loose lips sink ships ” was printed on posters and hung in many public places , such as schools and churches . This wartime expression warned people in the military as well as ordinary citizens to watch what they say . Unguarded talk may give useful information to the enemy . Part of the propaganda campaign warned that simply revealing the location of a loved one on a ship could be dangerous . This idiom still may not make sense to you . So , let's break it down . Something “loose ” moves easily . So , loose lips move freely and release a lot of words -- most likely in a careless way . The message : Be careful when you speak . Watch what you say . Beware of spies . And during war , you never know who is a spy . That is the nature of spying . One of the most famous – or maybe I should say infamous – spies in history was the Dutch dancer Mata Hari . Well , Mata Hari was the name she used when she performed as a dancer in Paris in the early 1900s . The online Encyclopedia Britannica explains that Mata Hari was romantically linked to a number of men . Many of them were military officers around the time of World War I . But when French officials suspected she was spying for Germany , they arrested her . She was tried and found guilty of supplying Germany with information . It is unlikely that Mata Hari was responsible for sinking any ships . And she may not have been a spy . Some documents reportedly prove her innocence . Although her relatives may find that evidence to be cold comfort -- after all , her reputation remains damaged . We still use the name “Mata Hari ” to describe a devious woman who uses her sexual appeal to get men to spill their secrets . Now , let's go back to loose lips sinking ships . Keep in mind that you do not have to be in the middle of a war to use that idiom . For example , let's say you are working on a project at work with a small team of people . Until you are ready to launch the secret product it is best that no one else knows . So , you can warn your teammates that “loose lips sink ships .” If they want the project to be successful , they should keep the details to themselves .