A person or activity that is considerably more extreme than hardcore. Usually describing the focus of an activity or person that is so intense, only the ones who disregard personal safety, friends, family, responsibilities and time to achieve it.
When the term hardcore doesn't define the experience or person well enough.
Spending 12 hours a day raiding for purple gear in WoW is diecore!
That kid is going to attempt a 720° backflip off that rail grind. He's not crazy, he's just diecore.