This question caught my eye because it's something I always wonder about LJ friends. As for me, I use Amarissia here because it's the name I use at fanfiction.net, so it's become my online name for anything yaoi. (The less naughty part of me goes by Athlynne, so if you see me on YouTube feel free to say hi, but kindly don't point and yell "I know you, you're the one who writes squicky yaoi!" Civilians are not prepared to handle that kind of thing.)
Anyway, there's no interesting story behind the name's origin. I needed a name, and it was the first thing to pop into my head. (And it starts with 'A', I seem to collect 'A' names.) I regretted it afterward. If I had known I would use this name so much, I would have taken care to choose something that doesn't make me think of something big and frilly and lacy, like a human doily.
Anyone else care to share?
ADDENDUM: Thank you all for indulging me!