I have moved this question to this group because it is in the Discourse that Rousseau truly addresses the idea of the noble savage.
To Rousseau, human nature is basically good. Rousseau believed that it was not people's evil nature that causes problems in the world. Instead, he believed that people started to have more problems as they moved away from the state of nature.
In the state of nature, Rousseau argued, people were noble savages. They were primitive and did not think much, but they were good and they were happy. People in that state, Rousseau says, are naturally inclined against making others suffer. They simply go along, living their own lives and not trying to hurt others.
It is only when people become "civilized" that problems start. People start to stake out property -- to differentiate between what is theirs (property, tribe, etc) and what is not. They then start to fight over these things and make elaborate socities based on having more things than other people.
So, to Rousseau, the noble savage is the human being in the state of nature. It is a savage because it has no civilization and no philosophy. But it is noble because it lives a good life, not trying to hurt or exploit others.