A long time ago,a carter was driving his cart,when a mantis jumped out in front of the cart. Raising its forelegs,it tried to block the way of the cart. The carter thought how ridiculous this inscet was,then moved on forwards. Easy to imagine,the fool but rather bold mantis was crushed by the rolling wheels. Nowadays,this idiom is used to mean overrating the ability of oneself and trying to do what is beyond one's power. The result can be nothing but failure.

很久很久以前，有一个车夫。一天，他正驾着马车在路上行驶。忽然前面跳来一只螳螂，举起镰刀一样的前腿，恶狠狠地挡住车轮的前进。车夫没有理会这只愚蠢的螳螂，径直往前行驶。当然，螳螂的身体很快被滚动的车轮压扁了。
后来，这个成语就被用来形容一些狂妄自大的人，他们想做远远超过自己能力范围以外的事情，结果是很凄惨的。

螳臂当车 (táng bì dāng chē)
【翻译】a mantis trying to stop a chariot.
【释义】当：阻挡。螳螂举起前肢企图阻挡车子前进。比喻做力量做不到的事情，必然失败。
【例句】这家企业规模小，技术落后，要以目前的实力，去和跨国大企业竞争，简直是螳臂当车。
【近义词】自不量力、泰山压卵
【反义词】量力而行、力所能及、自知之明