@Alucard already got mine, it's Japanese for (roughly) I don't understand.
I used it on tech forums because, well, if I'm posting question then clearly I don't understand something. Now I've come to like it. The avatar is the "sign of the void" that appears on a character in the manga "3x3 eyes" when his soul is devoured by the Sanjian (three eyed mythical Tibetan immortal), thus dooming him for ever to her service and to search the world for her human aspect whom he loves. I had it tattooed on my shoulder about 22 years ago...
Well, apparently I have been called upon to pull back the veil on the origin of my somewhat unorthodox username. lol I haven't had this particular pleasure in at least a decade and have forgotten it's vaguely polarizing nature over the years. Now that the site is back up, let's see what we can dig up.
Basically, it stems from the fact that my first name is Robert - hence Bob. Shocking, I know - but bear with me. Secondly, I've always been fond of obscure British slang of questionable taste. 'Nob' describes someone who is an idiot or a jackass - perfect for my self-deprecating nature. Thirdly, it rhymes! Witchcraft!
...of course, there is also the matter of the mischievously tongue-in-cheek sexual innuendo my name illicits. I'm sure some of you don't need that particular bit explained. That would ruin the fun, anyhow. It's multi-layered on purpose - I quite enjoy the duality of playing two hands concurrently. Like a poor man's mental fugue, if you will. Or high end schizophrenia.
There. The deed is done. Writ large in the binary blood of ones and zeros. Do with it what ye will, gentlemen (and ladies).
Mine isn't that original either. First name last name.
Back in the day I used to go by Bigus_Dickus (Monty Python reference) but it got me banned here and there. I changed to Wolfwood after one of my favorite anime characters in Trigun which is my favorite anime. Sadly is largely overused so I started using my own name.