I don't have to tell you ANYTHING!
But I may as well. It started simply as a joke, really.
I used to play Halo Combat Evolved on my computer , (Back when that was all I COULD play.) and it would say when THISPLAYER killed THATPLAYER. So, my name was "A Prius" But I couldn't use a space (Or something like that.) So I simply name myself "A_prius." That way, in mid-game, you'd see something along the lines as "0M6L337PR0 was killed by A_Prius."
So, that's one reason. Another,was that I named a few other gaming accounts Aprius, so my buddies from Halo could add me there as well. Soon it became my name for a lot of games, and forums alike.
Heck, every game I own now, including my offline Profile name for The Xbox360 (Screw Live at the moment.) is Aprius. Guitar hero bands? Aprius. Soul-Caliber home-made characters? Well, Kenny AND Aprius. On BakaBT I added Chan....'cause why the heck not?

But as for a 'real' nickname, I'm known around my school and friends, as 'Fido'. That started way back in 5th grade, where we had two Kenny's in one classroom.(Luckily it's cool, 'cause the other is my best friend.) Apparently, this blew the teacher's mind, so he asked us: "What do you like called?" Obviously, He said Kenny, and I HATE the names Ken/Kenneth, so I simply replied "Fido."