‘Barbarian’ was originally a term for someone who didn't speak Greek.

The Greeks used the term ‘barbarian’ against people from Northern Europe, because to the Greeks it sounded as though they were saying ‘bar-bar- bar’ when they spoke their own language. Compared to Greek, the other languages sounded like barking, and so they coined barbarian to describe the sound of the language and the people who spoke the language.

The term was further adapted in meaning as the Greeks began to apply it more frequently to the Northern Europeans. Because they were a nomadic people, the term barbarian also took on the meaning of a ‘wanderer.’ The term ‘barbarian’ also became more derogative because the Northerners raided Greek villages.

The term then meant not only a strange speaker and wanderer, but also a violent and uncivilized population. The fact that these new people could not read classic Greek, only compounded the Greeks' belief that they were uncivilized since they believed the Northerners to be illiterate.

(Source)