A shepherd was tending his flock near a village. He was bored and he wanted to have fun; he made a plan; he would play a trick on the villagers. How? He pretended that a wolf was attacking the sheep. So, he shouted out, “Wolf! Wolf! A wolf is attacking the sheep!” People in the village heard him. They came running, and the shepherd laughed at them. The villagers were frustrated. Then, on another day, the shepherd was bored again. He decided to play the same trick. He began shouting, “Help! A wolf is attacking the sheep!” The villagers heard him. They hesitated for a moment for they thought it could be a trick again.
But they could not risk the sheep. So, they came running again but there was no wolf. One day, a wolf really came and the shepherd cried, “Wolf! wolf!” But this time people didn’t believe him. They thought he was playing another joke. So they did not go to help him. And things were very easy for the wolf; he killed all the sheep one by one without a hurry. The villagers were very angry with the shepherd. They fired him (=he lost his job as a shepherd).
You cannot believe a liar even when he tells the truth.