Underdog comes from my days of my high school running a chess competition, which I only came across by chance on an after school detention - (I went to a pretty nerdy selective high school)
I asked one of the kids running it what was going on, and this particular lad who had a pretty dim opinion of me, acted surprised when I said I knew how to play chess - which was like waving a red flag to a bull - so of course I registered my interest.
This was no "ordinary" chess tournament - the boffins of 4 unit maths had even set up a betting racket with semi accurate odds -a crude setup where odds didn't change on the fly - however they were heavily favoured towards members of the chess club to ensure no one lost out - this part of the whole shindig was of course not known by the teachers.
I was put in the "unseeded" bunch of kids, and given 101-1 odds. I slapped 2 bucks on myself. What they didn't know is I was once a primary school chess prodigy, and although I had not played seriously in years, I knew that if I went home and resharpened my skills against the computer and a handful of old chess buddies I'd be at least semi competitive.
Needless to say 3 afternoons of chess later, I was $202.00 richer, and as I pipped the "Number 1 seed" in the semi final by grinding him down to King/Pawn vs King, before I swept the number 3 seed in the Final one of the lads watching said - "He was supposed to be comic relief! The underdog is never supposed to win!"
It stuck through the last year of high school, and I started using it for a username because the Interwebz was starting to get big(ish) for stuff like that - so ever since then I used it for most things, as it generally wasn't taken - so from 1999 onwards.
The rest as they say is history - I've pretty much used "Underdog" or some variation on it (underd0g, und3rd0g, underdawg, etc) when it's been in use.
Nerdy, and probably a bit boring and self serving - but as NPH says -
