Cry Wolf

This is Aesop's fable of the shepherd boy who repeatedly shouts that a wolf is attacking his flock purely to amuse himself, watching the villagers come running each time only to find nothing wrong. When a real wolf eventually shows up and he cries for help in earnest, nobody believes him anymore, and the flock is lost. The fable wasn't translated out of Greek and into wider European circulation until the 15th century, appearing in Heinrich Steinhöwel's collection of Aesop's fables, but the moral, that habitual dishonesty destroys your credibility even when you're finally telling the truth, has made the phrase durable across languages.