Hmm, oh God it was so long ago...story is long, silly and very immature, but then, so were we.
My friend once broke his finger while punching me in a shoulder. It was just a friendly punch, but he hit the bone and hurt himself. Being kids, we enjoyed watching X-men re-runs on Cartoon Network and that, coupled with the fact that I was the tallest kid in school probably made him yell "You're a freaking Colossus, man". When we started playing Half Life deathmatch and Team Fortress, I had to pick a name, so I went with Colossus.
In my high school days, when we started learning more about computers I found out that Colossus was the world's first digital, programmable computer. It was also used to break encrypted German messages during WWII, and I always liked learning about that period.
When I started playing online multiplayer (CoD 2 had just come out) I learned to my dismay that there are a lot of players with that nick, so I changed it to ClearScreen (we were studying basic console commands at the time), but I was missing the story behind my old name so I just shortened it and used uppercase CLS (CLS = ClearScreen). And so CoLosS came to be. Honestly, I have no idea why I put that zero in there.
Some people find the name awkwardly difficult to spell with all those uppercase letters and a zero, but I don't care. I like pissing people off in any small way. Also, it looks and sounds very silly.