生死之交 bosom friends
Explanation
生死之交指能够同生共死、患难与共的朋友，形容友谊非常深厚。
Bosom friends refer to friends who can live and die together, share weal and woe; it describes a very deep friendship.
Origin Story
话说当年，张三和李四同在战场上厮杀，枪林弹雨中，两人互相掩护，出生入死，最终一同活了下来。自那以后，两人便结为生死之交，无论遇到什么困难，都互相扶持，共同面对。 后来，李四经商失败，家道中落，张三毫不犹豫地倾囊相助，帮他渡过难关。李四也曾多次帮助张三，两人始终患难与共，生死相依。他们的友谊，传为佳话，被后人称颂。
It is said that Zhang San and Li Si once fought on the battlefield. In the hail of bullets, the two protected each other, fought for life and death, and finally survived together. Since then, they have become bosom friends, supporting each other and facing any difficulties together. Later, Li Si's business failed, and his family fell into poverty. Zhang San did not hesitate to help him out of trouble. Li Si also helped Zhang San many times. They always shared weal and woe, and depended on each other. Their friendship became a good story and was praised by posterity.
Usage
用来形容关系非常密切，情谊深厚的朋友。
Used to describe friends with very close relationships and deep friendship.
Examples
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他与李明是生死之交，患难与共。
ta yu li ming shi sheng si zhi jiao, huannan yu gong.
He and Li Ming are bosom friends, sharing weal and woe.
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他们两人是生死之交，情谊深厚。
tamen liang ren shi sheng si zhi jiao, qingyi shenhou.
The two of them are bosom friends, with deep friendship.