coffin nail

Something that might hasten or contribute to one's death, as in Every cigarette you smoke is another nail in your coffin. This expression, alluding to fastening down a coffin lid, is today almost always used for a harmful habit such as tobacco use (giving rise to the slang term coffin nail for a cigar or cigarette). The idea was first expressed in an ode by Pindar (the pseudonym of John Wolcot) in 1792: "Care to our coffin adds a nail, no doubt."

In Devonshire, England, superstition had it that a ring made from three nails or screws that have been used to fasten a coffin that was dug up in a churchyard would act as a charm against convulsions and fits of every kind.

The noun has one meaning:

Meaning #1: finely ground tobacco wrapped in paper; for smoking
  Synonyms: cigarette, cigaret, butt, fag