Good thread idea!
Way back in the late '90s, I had a problem on some sites with creating screennames that I didn't recognize when people were addressing me with it. For instance, I called myself "Chilly Down" on one forum and people kept saying, "I agree with CD," and I was wondering what compact discs had to do with anything.
So I decided that my next screenname would have my own real first name in it, so people could call me that for short and I would still recognize it, while still retaining some safety and anonymity.
This being the late '90s, I still loved George Lucas and enjoyed his frequent references to "THX" and "1138." So I came up with a screenname that is an amalgam of THX 1138 and my real name. It's worked well for me, so I've used it on a lot of sites and forums since then.
Ironically, I've never actually seen the namesake movie.