When I was a kiddo, one of my first words was mook; so I'm told I had a way of calling everyone a mook. From there on, the red fella' in the back of the Doom95 screen became a mook:
At that point, I was told about how it was one of my first words as a child and I thought it was rather amusing considering at that point (8 years old) the word came out naturally without me knowing exactly what it meant. Those cock suckers always killed me.
From there on I used it for my profiling, highscores and pretty much anything computer or game related. Once the internet filled up the variations started. Mook, M00K, Mookster, and as of late, Mookstah, and finally MookstahInc (a Sexyland based conglomerate dedicated to the enlightenment of ill-directed peoples, and encourager of natural selection -- Mookstah Incorporated.)