Originally this was the name of an award a player could recieve in Super Smash Brothers Melee © for not useing either R or L shoulder buttons which activated the shield during a fight.
It has since been addapted to other video games as a word/phrase to say after defeating someone soundly in some sort of game, normally a fighting game although some people use it for raceing games (and sadder yet... mario party v_v)
The word/phrase implies that the recipient had been beaten without even putting up a fight (the one saying it didn't even have to defend themselves or their position)
Player B: "damnit you suck"
Player A: "haha, NoR4U"
Player B: "Shutup, fag"