It's in reference to
Tel Megiddo. In particular, I chose the name because
Revelation 16:16 ("Armageddon" is thought to be Megiddo) highlights it as a major battlefield in the end times. There is also a B-movie about this battle called
Megiddo: The Omega Code 2.
Of course, no one knows for sure what will happen when the prophecy of Revelation comes to pass, but hey, I was only about twelve years old when I adopted the name. Speaking of which, I no longer go by "Megiddo" anywhere but in the garrysmod community. My preferred username these days is Nayruden.
Does that answer "what's a Megiddo"?