What did Thoreau mean by 'quiet desperation"?

Answer 1

he is referring to his idea that most of humanity suffers, maybe not physically, but spiritually because they deny (consciously or not) their connections to the world and everything in it...it is quiet because it goes on unnoticed and often unsaid...it is slow and persistant

Answer 2

Desparation refers to despair or loss of hope. So, Thoreau seems to be saying that most people have lost hope that their lives will be better or have meaning, and they do this without saying anything about it. They just trudge through their lives.