back when i was growing up, we used to call people beasts when they were good in sports or video games, or something.
"he is a beast at super mario kart, he can whoop me at it!"
and then we started calling the best of the best pure beasts.
"he is a pure beast at super mario kart, he can beat anybody!"
and then i just shortened it into one word, and you have purbeast. many people who see my nick (on these forums even) will type out purebeast or just refer to me as "pure".
Eniac is what many individuals consider to be the first electronic computer and added xa for some power and to freshen up the name.