coffin是“棺材”，another nail in your coffin字面看“又一根棺材钉子”，用来指代那些有害健康、危及生命的东西。人们给香烟起的绰号就是coffin nail.

【例句】
Every cigarette you light up is another nail in your coffin.
你每点上一枝香烟，就等于往坟墓跨近了一步。

Jim, you really ought to stop smoking. Every cigarette you light up is another nail in you coffin. I realize stopping is hard, but if I did it you can too.
Jim，你一定得戒烟，你每点上一枝香烟，等于往末日跨近一步。我知道香烟难戒，但是如果我能做到你也能。