A misspelling of the exclamation, "Nooooo". This word is often seen in the chat channel of an online game, where the speaker mistakenly hits the M and N keys simultaneously (which are next to each other in the common QWERTY layout) in the spelling of "Nooooo".
It is sometimes used as a taunt at LAN parties
>UberGamer smacked down LamerBob with the Rocket Launcher.
>UberGamer captures the flag!