Mine has a bit of an interesting story. Well, I think so, at least. It originated as Maxx, on the first site I joined. This was when I didn't want to be me anymore, with my real name, and Maxx is just an awesome name and an awesome way to spell it.
(I may have also drawn inspiration from the drummer of Set It Off, but oh well, lol.)
Anyway, Maxx was the perfect name. It was unisex, which meant I didn't have to reveal my gender or anything, and I kinda started seeing 'Maxx' as a separate person. They were this confident, friendly, cool person. The opposite of me. But slowly, I became more like that in real life, and realized "Wait, I should change my name to Maxx. It fits me much better than my actual name."
But this was all when it was still just Maxx, and I was still hiding a lot about me, even my gender. Though, being 'Maxx' did help me realize that my gender isn't binary, so it doesn't matter.
I can't remember when, but as I evolved as a person, I thought, Maxx should be short for something if I'm gonna change my name. That would be really cool. On my original roleplaying site, I became "Maxx[2.0]ine". I was v2.0 of myself, finally confident enough for that small hint about myself.
I weren't too happy with that spelling though. It was too normal. So I change 'ine' to 'een'. Partly because it makes my name a portmanteau (of sorts) of Maxx and Queen. And also just because it's awesome.
And that's where my username of Maxxeen came from.