It's the name of my Nob.
No, seriously.
I used to play Games Workshop wargames (Warhammer, Warhammer 40k). Games Workshop used to make a related spinoff game called Gorkamorka, and I played it quite a lot. You lead a group of Orks, called a 'mob', into battle with other mobs, and the Ork in charge of your mob is called the 'Nob'. 'Nazdakka' is the name of my mob's Nob - according to the back of the book it means 'Rich and Shooty' in Orkish.
When I was choosing a screen name for use on the internet, I was stuck for ideas. Then it hit me - why not use 'Nazdakka'? It's distinctive, and noone else is likely to be using it. And so I have been Nazdakka ever since.