work my ass off :【拼命工作break(或 bust)one's (sweet) ass: 拼命干】
I was working my ass off when my boss kicked back in the pub.
我玩命地工作的时候我老板竟然在酒馆里悠哉游哉。

on second thought: 仔细想想
I said I wouldn't do it, but on second thought I think I will.
我说我不会做此事，但再细想一下我想我是会做的。
He have intended to go, but on second thought, he give up the idea.
他本来想去，但进一步考虑後，打消了这个念头。