I know the dictionary definition of the word; it basically means uncultured and uncouth.
However, the use it gets is almost always when a group of one culture are invading and taking over a group of different people and the invaders need a reason to justify the fact that they are colonizing, enslaving, assaulting, and murdering a whole culture–so they call them barbarians and talk about how a group of barbarians would be entirely too stupid, uncultured, and uncouth to make their own civilization and they need the great invaders to help them.
That is how whites justified slavery, the genocide of native peoples, and their theft of land (and resources).
It’s also the way Charles Manson got his family to commit murder. He convinced them that by killing rich white folk, they could make it look as though the murders were committed by, specifically, black people and it would start a race war. Manson told them that the blacks would win but would be too stupid (too barbaric) to know what to do with the earth once they’d gained control and would turn to the only group of white people who survived the battle, the Manson family, hiding in their underground palace.
So…is barbarian a slur? It seems like a slur to me.
Not only is it a slur on its own, but it derives from a slur.
The Greek word barbaros means essentially the same thing, and comes from the equivalent of making ching-chong noises at an Asian person – “barbarbar” is how a lot of other languages sounded to them. It has been a slur in Latin and Old French as well, and has cognates in multiple other languages.