Mine is a shortened version of my regular username I use for everything, TearlessRain. Because FeralHeart did not like the length of it.
As for the meaning, I couldn't pick just one, which is probably why I've been using it consistently for so long (aside from the fact that it's gender-neutral and not related to a fandom, which is nice for long term). The simplest being that it works well as a metaphor for a meteor storm, and I have this thing for all that is space-related. I also love regular rain, so there's that. There are other weird reasons, but... meh, they don't make sense without way too much context. XD