cry wolf来自《伊索寓言》：《牧童和狼》（The Shepherd boy and the wolf）

有个牧童在离村子不远的山坡上放羊，有一次，他为了开心作乐，突然大喊：“Help! Help! The wolf!”全村的人都闻声跑来援助他时，才知道这只是开玩笑。如此恶作剧搞了两三次。后来，狼真的来了，那个牧童再呼号求救时，谁也不理会他了。于是，狼把他的羊吃了。

这篇寓言的意义很清楚：爱说谎话的人，即或在他说真话时，也没人相信他。由此，人们用to cry wolf 来概括这篇寓言的基本情节及其教诫意义，用以比喻to give false alarms; to warn of danger where there is none之意。

eg: Nobody will believe he is in trouble because he has cried wolf so many times.

Is she really sick of is she just crying wolf?

The newspaper placards that had cried "wolf" so often, cried "wolf" now in vain.