Why were the Celts viewed as fearsome warriors?

The Celts were known as fearsome warriors. They would fight naked and paint their entire bodies blue. They terrorized the Romans, even sacking Rome itself in the 4th century B.C. The meat of the back of the pig and the legs of the pig were so prized by the Celts that if two warrior disagreed about who should get that part of the pig, they would fight in single combat to decide the issue.