J.B. - Because my best friend in middle school decided I looked like Junie B. Jones.
Seka - A shortened version of my-real-name-that-I'm-not-telling, that I adopted when I was little because it sounded more Samoan than my-real-name-that-I'm-not-telling, and since I was the only palagi (white) kid in our huge extended family of Islanders, I needed all the help I could get fitting in.
That Weird Kid - Everyone who worked in the buildings behind my mom's store or even just visited there. I... have an over-active imagination, and used to have this habit of going into the parking lot behind my mom's health food store while she was working, and acting out various scenes from what ever story ideas I happened to feel like enacting at the time, be they battles, chase scenes, or what have ya. (A lot like Krom, actually, except it wasn't restricted to just smiting enemies, and, you know... I still do it.) Unfortunately, those people in the building behind us didn't know what was going on in my head, so if they happened to look out their windows, they just saw a kid walking/skipping/running around in circles in the parking lot, talking to herself. It became especially amusing when a church moved back there.
Jani Rieme - My first internet pen-name. And no, it's not pronounced "Johnny", or "Jah-nee"... The J is pronounced like the X in "Xiao". "Jani". And don't ask what it means, because I haven't the faintest clue, I just made it up one night.
Kuro Shinzui - My second internet pen-name. This was spawned when I went through the *ahem* obligatory "weaboo" phase all writers of fanfiction for anime seem to go through.
Psychosis/Psy/Psychosis L. Neurotica - My longest standing nickname and the one I still use. Started as just Psychosis, eventually tacked on the "L. Neurotica" just for shits and giggles. And the short version is NOT "Psycho", it's "Psy" for all you lazy people that can't be bothered to write "Psychosis"!
Strayscream - A very recently developed nickname. This one came about when I joined the Transformers fandom and started RPing with this one chick- she introduced me to her friend, who, for no apparent reason, called me "Strayscream". I pestered my partner for the story, and finally she explained that they have a group of RPers, and their former Starscream-player was, er... troublesome? Moreso than the character she actually played, anyway. *snickers* So they've been looking for a new Starscream player, and she was starting to despair of ever finding a good one, when one day her friend said "Hey, don't worry, I'm sure we'll find a little stray Starscream roaming around sooner or later!" And then the next day she met me, and the nickname "Strayscream" stuck.