1. A shortened, ungrammatical version of: "I am so violently pissed that I am going to kill you!"
2. A death threat*.
*Person issuing threat is often too angry or furious to use proper grammar. Usually followed by violent act.
The definative definition after twenty years of hearing it used in context: "I command you to eat shit (which is the most putrid experience one can imagine as a living being) and then die (which many view as the most undesirable state of being--curiously worse than eternal torture, unless dying and going to hell are synonymous). It represents an ill feeling toward another where simply eating excrement does not do justice to offense given, nor is dying: both must be experieced to satisfy vengence.
The quintessential insult, the godfather of all putdowns, and a drunken sailor's best friend; it leaves no room for reconciliation.
Eating shit is bad enough (can you think of anything worse?) but wishing DEATH upon someone? That is treading in shark infested waters. Be wary, my son; utter these words only in the darkest of hours, when all other aspersions fail you.