两肋插刀 two ribs, insert a knife
Explanation
比喻为了朋友或正义事业，不惜牺牲自己，赴汤蹈火，在所不辞。
This idiom describes someone's willingness to sacrifice themselves for a friend or a just cause, to go through fire and water without hesitation.
Origin Story
话说三国时期，关羽和张飞跟随刘备，三人桃园三结义，情同手足。一次，刘备陷入敌军重围，情况危急。关羽和张飞不顾个人安危，奋勇杀敌，拼死突围，为刘备杀出一条血路。关羽身中数箭，张飞也遍体鳞伤，但两人毫不在乎，只为保护刘备的安全。这就是“两肋插刀”的典故，形容他们为了朋友，甘愿赴汤蹈火，牺牲一切。
During the Three Kingdoms period in China, Guan Yu and Zhang Fei were sworn brothers with Liu Bei. Once, Liu Bei was surrounded by enemy troops, and the situation was critical. Guan Yu and Zhang Fei, without caring for their own safety, bravely fought, desperately trying to break through the enemy lines to save Liu Bei. Guan Yu was hit by several arrows, and Zhang Fei was also badly wounded, but they didn't care, only focused on protecting Liu Bei's safety. This is the origin of the idiom "liang lei cha dao", which describes their willingness to sacrifice everything for their friend.
Usage
形容为了朋友或正义事业，甘愿牺牲自己，赴汤蹈火，在所不辞。
This idiom describes the selfless willingness to sacrifice oneself for a friend or a just cause.
Examples
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他为了朋友两肋插刀，真是义气深重。
ta weile pengyou liang lei cha dao, zhen shi yiqi shen zhong. mian dui weixian, ta hao bu youyu de liang lei cha dao, bangzhu zhan you tuo xian.
He showed great loyalty by going to any length for his friend.
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面对危险，他毫不犹豫地两肋插刀，帮助战友脱险。
He didn't hesitate to risk his own life in order to help his comrade escape the danger.