Refers to a can of chewing tobacco. A stereotypical coffin is full of death, as so is a can of chewing tobacco. But the word is used amongst chewers as a code so that they may talk about their tobacco with out any having to know.
"dude, i'm dyin right now...we need to get a coffin fast"
"Hey does anyone have a coffin?"
"what does it take for a man to get a coffin around here?"