Gosu (고수) is a Korean term used to refer to a highly skilled person. In computer gaming the term is usually used to refer to a person highly skilled in multiplayer games like StarCraft, Warcraft III, DotA (RTS/RPG), Diablo II (RPG), Counter-Strike (FPS) and others. The term was adopted by gaming communities in many countries because of a large South Korean presence in online gaming communities.
Source: http://en.wikipedia.org/wiki/Gosu