Mine is my nickname in real life and the "101," moniker looks/sounds cool (to me), but I'll explain the underscores.
So in grade 6 (when I was still 3rd tallest of my peers and was fairly-decent at basketball), when defending my buddy who wasn't the greatest at basketball, but knew the sport like I know baseball and F1, I didn't know the proper defensive concepts, so I just charged and tried to steal the ball, always. That, combined with my size, he said, "you're like a truck!" (in an endearing way).
He then started calling me that and lo and behold, more people started calling me that lol. Then, I made the mistake of introducing myself in EVERY grade 9 class as, "my nickname is Truck," and I got called that more than my actual name, by my peers lol (my closest friends always called me by my first name though).
In 2003, my 2 closest friends and I started playing Age of Empires 2 on MSN Gaming Zone (lol); one of said friends and myself, had been playing the game since it came out (he's better than me), but the other friend was new to it, so we decided to have a few rounds. When creating a Gamertag, obviously, "Truck," was in use, so I tried, "Truck101," it too was in use. So I played around with names, until "Truck_1_0_1_," came out and it looked good (to me lol), so I went with that!
Thus, it is my username, gamertag, etc. etc. I literally use it for everything:
gamefaqs.com/features/recognition/61030.html
And see gamertags under my name :)