I was a pretty strange kid in middle school and would do just about anything to make people laugh. One of the things that stuck was shouting out the word Argabarga! I'm also a big fan of Indiana Jones.
Hell I still don't know if it means anything, but I like the way it sounds and I've been using it since I started playing Quake II. There also aren't a lot of other people using it as a screen name out there.