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.