The Boy Who Cried Wolf, also known as The Shepherd Boy and the Wolf, is a fable by Aesop.
The protagonist of the fable is a bored shepherd boy who entertained himself by calling out "wolf". Nearby villagers who came to his rescue found that the alarms were false and that they'd wasted their time. When the boy was actually confronted by a wolf, the villagers did not believe his cries for help and his flock perished. In another version the boy was killed by wolves.
The moral is stated at the end of the fable as:
Even when liars tell the truth, they are never believed.
The English idiom "to cry wolf", derived from the fable, refers to the act of persistently raising the alarm about a non-existent threat, with the implication that the person who cried wolf would not be taken seriously should a real emergency take place.
On the TV series, Star Trek: Deep Space 9, the Cardassian Garak has a different understanding of the story, suggesting its message is "Never tell the same lie twice". On the Simpsons episode "Marge Gets a Job", Bart Simpson indeed tells not one but several different lies to avoid taking a test, only to literally be attacked by a wolf and, subsequently, not believed.
A cynical interpretation is also possible: Do not lie too often, and do not tell lies just for your own amusement; save lies for when they are urgently required.
參考資料:
http://en.wikipedia.org/wiki/The_Boy_Who_Cried_Wolf