Used to be Kestrel, based on the bird of prey. I like things that fly.
Kestrel wasn't taken very often on boards, but often enough to be annoying. I ended up on yahoo with the username Kestrel with like a bunch of underscores in front AND behind the name.
Some time later, Coppertop and I were trying to think of clan names for Team Fortress (way back when) and Coppertop threw out the acronym FNG. I liked the acronym enough to put it at the front of Kestrel.
After adding that to my name, the best compliment I ever got in Team Fortress was some guy on the opposite team saying, "Dude, you don't play like an FNG."