I'll just have robin crack my back when she gets home.

我让罗宾回来给我捶背就行了。

I'll crack your back. -What? No.

我帮你舒展舒展 -什么？不。

He fell backwards and cracked the back of his head on a lamp post.

他向后倒去 后脑勺撞到了路灯柱的底座。