the.com/coffin nail

means An old slang term for a cigarette, named with blunt honesty for what it eventually does to you.

from Emerges in American slang around the turn of the 20th century, when 'coffin nail' and the shorter 'nail' were already common tough-talk for cigarettes — folk wisdom casting each smoke as one more nail driven into your coffin. It caught on fast in newspapers and pulp slang dictionaries, riding the same wave of gallows humor that gave us 'cancer stick' decades later, long before health warnings made the joke official.