My nick? Uhh, I thought it sounded cool.
And it kinda resembles my first name: Maarten.
I chose it just before the movie The Matrix was widespread known, including myself. The first time I used it was because I needed a nick when becoming a member at a Laser Quest club on january 21st 2000. So everybody thought I was unoriginal and I chose it because of the movie.
What it really means is: mathematical table. And Schwarzenegger was "Colonel Matrix" in Commando.
Those are the two associations I had when choosing it. But the real reason is: I thought it sounded cool.