My first name is a very common one, at least in my area. So naturally, when I'm in a large group, eventually there'll be one other person with the same name, so to differentiate, we tend to give nicknames. In my Boy Scout troop, we would call me Batman, since I once got a sunburn on my chest in the exact shape of the bat symbol, and my Eagle Scout project involved erecting bat houses in a local park.
In my Smash Bros. group, we call me Banana. The story behind it actually goes back to around 2008, when Charlie the Unicorn 2 had just come out. A few of us were hanging around and playing Melee, and since I was one of the two best among us at the time, they had me face the other best one-on-one to see who would win. They decided that the winner would be crowned the Banana King. Well, I won, and I started using the tag BNNA in Melee ever since. I think the name Banana has just the right amount of silly to suit me.