A thousand 钧 (jūn) hanging from a single strand of hair (see Notes section)
Refers to a something that is extremely critical or dangerous
Neutral
在这千钧一发的时刻，他勇敢无畏的挡住了敌人的子弹 | Zài zhè qiānjūnyīfà de shíkè, tā yǒnggǎn wúwèi de dǎngzhù le dírén de zǐdàn
At this perilous moment, he bravely obstructed the enemy's bullets
[Source]
他在千钧一发之际逃过那场大火 | Tā zài qiānjūnyīfà zhī jì táoguò nà chǎng dàhuǒ
He escaped from the fire at the critical moment
[Source]
None
一发千钧 (yī fà qiān jūn)
A 钧 (jūn) is an ancient unit of weight. It's roughly equivalent to 33lb (15kg)
This chengyu first appeared in 《列子》(Liè Zǐ), but the story itself is derived from when the expression was later used in《汉书》(Hàn Shū), a work documenting the history of the Western Han dynasty (西汉, Xī Hàn, 206 BC - 8 AD).
Note that 发 (fa) is fourth tone here, not first
The period of chaos described in this story is known as the 吴楚七国之乱 (Wú Chǔ Qī Guó Zhī Luàn), or "The Rebellion of the Seven States."
《列子》(Liè Zǐ), a set of Daoist texts said to be by the author of the same name. He is said to have lived during the Warring States period (475 - 221 BC)
During the Western Hàn Dynasty (西汉, 206 BC - 8 AD), there were many states that pledged loyalty to the Hàn emperor but still enjoyed a relatively high degree of autonomy and power. These states were ruled by Hàn princes, and as time went on and the princes became more powerful, a few of them started to challenge the authority of the emperor, 汉景帝 (Hàn Jǐng Dì). The emperor attempted to reassert his control by stripping away parts of the states' territory, but unfortunately, this measure backfired, and seven princes started to collude with one another to overthrow the throne.
The head conspirator was 刘濞 (Liú Pì), the prince of Wu (吴, Wú). In his service was a famous poet and official 枚乘 (Méi Chéng). When 枚乘 heard of the planned rebellion, he grew extremely concerned. He wrote a letter to 刘濞 and urged him not to rebel, saying, "To go against the Hàn court would be like trying to suspend a thousand 钧 (jūn) high in the air above an abyss with a single hair. Even foolish people know that to do such a thing is too dangerous!"
Ultimately, though, 刘濞 and his co-conspirators did not heed the poet's warnings. Their rebellion was crushed, and 刘濞 was killed.