千钧一发 Hanging by a thread
Explanation
比喻情势危急，如同千斤重物悬在细小的头发上，稍有不慎就会坠落。
It describes a situation that is extremely dangerous, like a thousand-pound weight hanging on a single strand of hair, which could fall down at the slightest carelessness.
Origin Story
唐代诗人李贺，有一次外出游玩，在山间小路上遇到了暴风雨，天色昏暗，山路泥泞，李贺骑着毛驴，在风雨中艰难地行走，眼看就要被大雨冲下山坡，突然，一只乌鸦飞到驴子的背上，李贺惊慌失措，用手去赶乌鸦，可是乌鸦就是不走，最后，李贺的驴子终于在乌鸦的帮助下，安全地走下了山坡。
Li He, a poet of the Tang Dynasty, was once traveling on a mountain path when he encountered a violent storm. The sky was dark, the path muddy, Li He rode a donkey, and struggled through the storm. He was about to be washed down the slope by the heavy rain, when a crow suddenly flew onto the donkey's back. Li He was terrified and tried to drive the crow away, but the crow stayed. In the end, the donkey was able to safely walk down the slope with the help of the crow.
Usage
这个成语用来形容事情到了极其危险的境地。
This idiom is used to describe a situation that has reached a point of extreme danger.
Examples
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这场比赛真是千钧一发，最后时刻我们才绝杀对手，取得胜利！
zhè chǎng bǐ sài zhēnshi qiān jūn yī fà, zuì hòu shí kē wǒmen cái jué shā duì shǒu, qǔ dé shèng lì!
This game was really close, we only won in the last minute with a winning goal!
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他开车冲下悬崖，千钧一发之际，抓住了一棵树枝，才没有坠落下去。
tā kāi chē chōng xià xuán yá, qiān jūn yī fà zhī jì, zhuā zhù le yī kē shù zhī, cái méi yǒu zhuì luò xià qù.
He drove his car off the cliff, at the last minute he grabbed a tree branch and didn't fall.
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这个项目已经到了千钧一发的时候，我们必须全力以赴，不能再有任何失误！
zhè ge xiàng mù yǐ jīng dào le qiān jūn yī fà de shí hòu, wǒmen bì xū quán lì yǐ fù, bù néng zài yǒu rè hé shī wù!
This project has reached a critical point, we must go all out and can't afford any more mistakes!