Coffin nail is a common piece of slang for cigarettes. It's been around since at least the 1920s, and some sources say the 19th century †; in any case it's been around the block once or twice and is now a well-known term, included in most dictionaries and generally recognizable even by nonsmokers. One online dictionary helpfully describes them as "finely ground tobacco wrapped in paper; for smoking" for people who have never actually seen such a strange and outlandish thing before. For those of you with particularly poor interpretative skills, I will point out that, as cigarettes are both long and thin (like a nail), and more smoking is pretty clearly coorelated to faster dyin', one might say that a smoking a cigarette is 'putting one more nail in your coffin', or something along those lines. Just thought I should clear that up for you.
Besides the basic definition, there are two things I want to discuss about this. First, coffin nails is a fucking cool thing to call smokes. Whenever I think about it, I just want to smoke a pack a day, and eventually die of throat cancer like Bogie, or go get into a knife fight and pretend I'm James Dean. Presumably my intense desire to do things that will kill me is some sort of Darwinian thing to keep me from reproducing. Or something.
Secondly - since finding out that this term has been around for at least 70 or 80 years, I am starting to get very suspicious of the recent spate of lawsuits claiming "I didn't know cigarettes would kill me." They're not called coffin nails 'cause they're good for you, morons! Anyway, I'm not meaning to fill this node with an argument about this, and I know probably not everybody knew the term when they started smoking.
/me wanders outside for another smoke...
† Merriam-Webster says 1888.