A term used in online discourse to belittle and bully others. Often an expression of impatience with a game-player who might seem less than proficient, but also just a handy insult for those without the vocabulary, manners, humour, tolerance or spelling that their mums hoped they'd have by that age.
A: Oh, bother. We lost the game, possibly because of an error on the part of player B. My life extends not much further than the online game world, so obviously I'm miffed. How shall I express my frustration? With a cheerful and sporting, "We'll get 'em next time" or "Don't feel too bad"? Nah! Hey, player B: "n00b!" That told him.