Once a carter(车夫) was driving his cart, when a mantis(螳螂) jumped out in front of the cart.

Raising its forelegs, it tried to obstruct the passage of the cart.

Of course, it was crushed by one of the wheels.

This idiom is used to mean overrating(估计过高) oneself and trying to do what is beyond one's ability.

The result can be nothing but failure.

春秋时，卫灵公请颜阖为太子之师，颜阖知蒯瞶为人阴险凶残，欲改其恶习，导之正途，但恐遭杀身之祸，而问於蘧伯玉。

伯玉语喻之曰：螳螂奋其臂以挡车轮，其不知否能胜任也，徒取灭亡耳。

不可高估本身之力，而为太子之师。

若自持己见，必遭太子之嫌。

是以务必小心从事。

”颜阖从之，婉拒卫灵公之请。

亦作螳臂挡车”或作螳臂当辕”。

【中国成语故事中英对照 螳臂当车查看网站：[db:时间]】