命在旦夕 life hanging in the balance
Explanation
形容生命危在旦夕，随时可能死去。旦夕，指早晨和晚上，形容时间极短。
Describes a life hanging in the balance, possibly dying at any time. Danxi, referring to morning and evening, describes a very short time span.
Origin Story
战国时期，名医扁鹊为魏王诊脉，发现魏王身体抱恙，命在旦夕。扁鹊告诫魏王，若不及时调理，恐有性命之忧。魏王不信，以为自己身体康健，直到病情加重，才意识到情况危急，可惜为时已晚。
During the Warring States period, the famous physician Bian Que examined the pulse of the King of Wei, and found that the king was unwell and his life was in danger. Bian Que warned the king, if he did not treat it in time, there would be a risk to his life. The king did not believe it, thinking that his body was healthy, until his condition worsened, he realized the situation was urgent, but unfortunately it was too late.
Usage
作谓语、定语；形容生命垂危，情况紧急。
As predicate, attributive; describes a life hanging in the balance, urgent situation.
Examples
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战士们个个带伤，但依然坚守阵地，他们的生命正处在旦夕之间。
zhànshìmen gègè dàishāng, dàn yīrán jiānshǒu zhèndì, tāmen de shēngmìng zhèng chǔ zài dànxī zhī jiān.
The soldiers, though wounded, remained at their posts, their lives hanging in the balance.
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大火烧毁了大部分的房屋，村民的生命都危在旦夕。
dàhuǒ shāohǔi le dà bùfen de fángwū, cūnmín de shēngmìng dōu wēi zài dànxī.
A great fire destroyed most of the houses, and the villagers' lives were hanging in the balance.