Atomsk is originally a spy novel by Carmichael Smith, known for its suspense, its a lot like James Bond.
Atomsk was later used as the name of a character from the relatively popular anime FLCL (short for Fooly Cooly) He is called the pirate king due to his ability to envelop entire solar systems. He looks like a giant red bird with a wingspan of an entire city, he has a nosering and no definite eyes. Pretty damn awesome.
Finally, Atomsk is a Hip Hop artist from Portland Oregon. He started out doing Nerdcore Hip Hop (basically rap about nerdy stuff) and has since removed himself of that label. He takes his name from FLCL said to be his favorite anime, and he represents this name well. He also goes by Atom for short.
One interesting thing is that there is no real correct pronunciation of Atomsk. However the director of FLCL decied to go with AT-uh-musk, which is how Atomsk the hip hop artist pronounces it.
Atomsk had the hottest verse on that track.