I don't think this particular question has been asked before, so I figured I'd start a thread about it. I'll go first (obviously, lol): Mine is the user-part of the email address I chose for my first e-mail address back when I was 13. I created the e-mail address alexschrod@hotmail.com (now defunct) based on my name, which is Alexander Schrøder, except I had to replace ø with o, since the Internet didn't like non-ASCII characters (and often still doesn't, actually) back then.
Mine originates from Red Dwarf (BBC late 80's - 90's), thus the picture , the ZBSD comes from being a donator (D) for ZeroBullS**t TF2 server - now defunct - and we started a Minecraft server back in the early beta days and that is what I logged on with Mojang initially
Mine is from a nickname "Jimbo" I used to say "that's Jimbo to you sir and Nothing else" I had a Game Center called Jimbo Nothing Else and I just shortened it! 64 is just a number that sticks in my head
Well, one day my friends and I were traveling to a theme park and were hungry. So we stopped at a McDonalds. At the time it was breakfast our. So we all told my friends mom what we wanted, and when it came time for me to tell her I said "Five hashbrowns". They dropped dead laughing since I am only 90lbs at the age of 14. So from then on I was called hash everywhere I went.