'Another nail in the coffin' literally means another step nearer to death or the end. A coffin is the box in which you are buried when you die. So each step that a person takes may be destroying their future or career os something similar. Hope this helps Bob

There is one very specific idiomatic use you might run into if you read books from around 1900. _Cigarettes_ were called "coffin nails!" It was very common. An example of use occurs in the 1906 O. Henry story, "The Higher Abdication." One character asks another for a cigarette by saying "Say, sport, have you got a coffin nail on you?" The phrase probably was a reference to _morality,_ not health! Cigarettes were thought to be, somehow, immoral... like drinking, I suppose, or like marijuana today. The writer Owen Johnson wrote many stories set in the Lawrenceville School, a private secondary school that prepared adolescents for Princeton University. In his stories, the older students are always secretly smoking and the younger students are always trying to smoke and getting sick. One of the schoolmasters gives an impassioned speech against smoking and says: "I tell you, now is the time to think of your mother; now is the time to spare her gray hairs. Every cigarette you boys smoke is a nail in the coffin of your mother!" In other words, if your mother found out you were smoking, it would kill her.

Simply put, it's the idea that things are getting worse. It implies that a person has already done things wrong (hence 'another') and now has just made things even worse.