Literally, "killing of pain," it is what most call suicide. It refers to the fact that those who kill themselves don't truly want to die, just to release their pain. I must say, though, it seems a bit superfluous to me - after all, their intentions regardless, they do, in fact, kill themselves. "Penacide" might as well mean getting blind drunk in order to kill one's pain, as well. But that's just my opinion.