从字面上来解释，to give you the shirt off his back 就是把他的衬衣从背上脱下来给你。一个人要是肯把衬衣脱下来给你的话，那就是一个在你患难的时候能拿出他所有的一切，包括他的衬衣，来帮忙的人。这种朋友是很可贵的。所以，to give you the shirt off his back指尽自己的力量帮助别人。
例如：Of course Dad gets mad at me sometimes for no good reason. But if I ever need it, I know he would give me the shirt off his back.
我父亲有时对我发火也不见得有什么道理。但是，只要我需要，他总是会尽一切力量帮助我的。
又如：Pete never says very much. But when he hears about somebody who's sick in the hospital or lost his job, he's always the first to put his hand in his pocket and bring out money to help. Yes, sir, he's the kind of man who'd give you the shirt off his back.
皮特这个人平时话不多。但是，每当他听说某人有病进了医院，或失了业，他总是第一个拿钱出来帮忙。是的，他是这种会尽力帮助别人的人。