Barbarian is an insulting word for a person from an uncivilized culture or a person with no manners. Barbarians aren't known for their etiquette.
Those barbarians — back in ancient times they were always invading and pillaging and generally unleashing their fury on the more "civilized" Greeks and Romans. The barbarian hordes are long gone, but we still use this word as an insult for anyone who's acting rude, uncultured, or particularly savage. If you pick up a whole turkey leg and start gnawing like an animal at the dinner table, the other guests might call you a barbarian.