临阵磨刀 sharpen one's knife at the battlefield
Explanation
比喻做事没有准备，到事情紧急的时候才匆忙应付。
The idiom refers to those who do not prepare well and only take hasty action when the situation is urgent.
Origin Story
话说古代，有一位将军，率领大军去征讨叛乱。这位将军平时不重视训练，也不做充分的准备，等到与敌人交战时才发现自己队伍的战斗力远远不如敌人，兵器也老旧不堪，于是下令士兵们赶紧磨刀磨枪，但为时已晚。结果，军队大败，损失惨重。这个故事后来就演变成了成语“临阵磨刀”，用来比喻做事不讲究准备，临到关键时刻才慌忙补救，结果往往事与愿违。
In ancient times, a general led his troops to fight a rebellion. The general did not value training and preparation, and when he fought against the enemy, he found that his troops were far inferior to the enemy's in terms of fighting ability and that the weapons were out of date. He hurriedly ordered the soldiers to sharpen their weapons, but it was too late, and his army was defeated with heavy losses. This story evolved into the idiom "临阵磨刀", which is used to describe the actions of those who are ill-prepared. They rush to make amends at the last moment, often in vain.
Usage
用于比喻做事不讲究准备，临时抱佛脚。
Used to describe someone who doesn't prepare adequately and is only working on it at the very last minute.
Examples
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这次考试，他临阵磨枪，结果还是没考好。
zhè cì kǎoshì, tā lín zhèn mó qiāng, jiéguǒ háishi méi kǎo hǎo
He sharpened his knife before the battle, only to fail the exam.
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不要临阵磨刀，平时就要认真学习。
bùyào lín zhèn mó dāo, píngshí jiù yào rènzhēn xuéxí
Don't wait till the last minute to study; make it a habit to be prepared beforehand.