Initially it was simply two words that I thought had a very contrary meaning together. When you think Ninja, you usually don't think "happy". Also I've found that whenever I come up with a username online, and I get the "sorry that name's been taken" message, if I put 42 at the end of it, it was always approved. So now I incorporate it into most of my names as a preventative measure against that message.
Since creating the handle though, I invented a story for it, inspired by Dr. McNinja.
I was once a member of a Mook Ninja Army, sent to do battle against an Epic Badass Hero. Due to the Inverse Ninja Law, me and my compatriots were summarily cut down to nothing, *insert dramatically epic music here* Until only I remained! And no, as the Sole Survivor Ninja, I was no longer subject to the Inverse Ninja Law, and had power leveled up to being en Epic Badass Hero myself!! *triumphant horns play here* I did battle with the man who slew my comrades and crushed him!! But in honor of my fallen friends, I kept my old number of 42 to pay tribute to their sacrifice. *tragically nostalgic music at the loss of loved ones concludes here*
The End