Noooooo (the number of o's vary) (pronounced with a Y sounding syllable [Nyooooooo]) was a running gag of Nova, used in any situation where he wanted to childishly protest something. The gag spread to the Creatures.
The joke was started by Nova in his MINECRAFT FINISHED 1.0.0 rage video, in which his mother attempted to take away his keyboard because of his "bitching" and yelling. Nova protested by whining "nyoooooo" and telling her she was taking Notch's side. The phrase has appeared a lot after this, including in Creature Minecraft videos and Creature Talks. The phrase has not been used often in more modern content, but will always be remembered as being an iconic quote.