Krass is a person who is very skilled in something. The words origin goes back to a talented Counter-Strike player who lives in Germany and uses several fake nicknames on LANs, while he hides in the darkest corner of the room, so that no one recognizes his presence. Some people also use his real first name "Jannik" as a synonym for krass.
Girl: Who are you ?
Boy: I am krass !
Girl : I did not ask what you are. I asked who you are !
Boy: Yes, as I already mentionend, I am krass.
Krass is a German slang word referring to the intense aura farming awesomeness of Kinnikuman Phoenix, Itachi Uchiha and Roronoa Zoro amongst other Shonen Jump hotties.
Krass may as well be the best word to describe how ironically incredible a Menace Stu like Tarzan or Luffy can really be.