The name is a riddle of mine. I wouldn't usually answer, but I'll tell you since you've been politer with me than the ones who accused me of being a troll and all kinds of other things.
As you know, Jahannam is Arabic for "hell" and Allah is Arabic for "God". Therefore, although western Christians identify "Allah" as the Islamic god, actually Arabian Christians also refer to their God as Allah, and Jesus as the "Son of Allah".
By the same token, Jahannam is hell in the Qur'an, but could also refer to the Christian hell. In other words, words like "Allah" and "Jahannam" which at first seem heathen, are in fact Christian concepts- it is not the word itself, but the way they're used that matters. Take the inverted cross of St. Peter which Christian condemn as "being of Satan"; actually the inverted cross was a Christian symbol for two thousand years, it's just that some Satanists came along and tried to steal it for themselves. Same thing with the rainbow- the rainbow is the symbol of God's promise to Noah, not "gay rights". What's important isn't the word or symbol itself, but how it's used. Christians too often judge something as "evil" which is, in fact, good.
That is the first reason for the name.
The second reason is this: the fires of hell do not burn innocent people. Hell-fire only touches the wicked. Hell-fire is the great fear of Satan and demons. Therefore, what at first seems "evil" is, in fact, the truest expression of God's divine justice. Therefore, if the flames of hell were living flames, then that spirit would be the truest expression of God's justice, because it condemns what is evil but does not harm what is good. The Bible also speaks of an intense "fiery trial" that cleans and cleanses, again- another fire which destroys evil, but this fire purifies what is already good and makes it better, like "refined silver" (forget the exact verse).
Thus, "spirit of Jahannam" doesn't mean a spirit dwelling in hell, it means a spirit that is an expression of hell-fire itself.
This isn't to say I'm trying to make myself out as something great and wonderful, as some would assume, just that I approve of the concept.