It means that the only way you can be sure that something remains a secret is to tell no-one. If another living person knows the secret, you can't be sure it won't become more widely known.
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Hollow Can somoene explain me this sentence :"Two can keep a secret if one of them is dead" . (IfI would think that they were refusing to tell me theirsomoenesomeone tells you that, what would you think?)