natalief was my first ever online (on JANET) username in 1985, defined by my then name Natalie Fricker. It was my username at college (both York for my BSc and Kingston for my MSc) and then was my username at most employers as well. I think I may have been nfricker somewhere but that did not stick or get used as often.
When I married hubby, my surname changed from Fricker to Ford and so natalief still worked as a username at work and elsewhere. I try to get natalief as a username wherever I sign up but occasionally (e.g. Ravelry) that has already been taken and I then tend to use natalieford (same as my web domain, natalieford.com) or nataliesford (my middle initial is S).
I have only rarely used a pseudonym that was not based on my real name and cannot remember where I may have done this. I know that I was Delenn once (having watched a lot of Babylon5) but cannot remember any others. To be totally honest, I do not have enough imagination to come up with pseudonyms. I have a couple of pen-name/pseudonym LJ accounts reserved but these are all names based on family names from the past and so are still not that imaginitive.
So, I suppose that you can say that the origin of my username is my real-life name and my online history.