One of Aesop's fables. A young shepherd would trick his fellow villagers by shouting for help, pretending that wolves were attacking his sheep. Several times the villagers rushed to his aid, only to find the shepherd laughing at them. One day, some wolves actually came. The shepherd cried for help, but the villagers, who had grown tired of his pranks, ignored him, and the wolves devoured his sheep.
Note: To “cry wolf” means to issue a false alarm.