铁哥们儿 (tiě gēmenr) close friends; faithful pals; buddy

Usage:

“铁哥们儿”的字面意义是关系像铁一样坚硬的兄弟，是指关系特别好的朋友，一般用来形容男性。
“铁哥们儿”literally means the relation between brothers is as hard as iron. It refers to close friends, usually male friends.
Sentences:

1、Xiǎolǐ shì ge hěn hǎo xiāngchǔ de rén. Zài yìqǐ gōngzuò yígèyuè zhīhòu, tā gēn quánzǔ de rén dōu chéng le tiě gēmenr.
小李是个很好相处的人。在一起工作一个月之后，他跟全组的人都成了铁哥们儿。
Xiaoli is an easy-going person. He became the close friend of all the team members after working together with them for a month.
2、Fàngxīn ba, tā shì wǒde tiě gēmenr, yídìng huì jìn quánlì bāngzhù nǐ de.
放心吧，他是我的铁哥们儿，一定会尽全力帮助你的。
Don’t worry. He is my buddy, who will do his best to help you.