An enthusiast of any video game, namely SSBB (where the word originated), who is naturally skilled at the game. They neither rely on countless hours of practice nor guides on how to become a worthy opponent, but by a natural ability to be above average.
Of course, a Nexro is not automatically the best; they merely learn faster and perform better than the average gamer.
"He barely ever plays this game, and he's still better than me! He must be a nexro!"