Nubia was the name of a protohistorical kingdom or region in East Africa, famed among contemporary North African and non-African people they had contact with for having really good-looking women compared to the rest of Sub-Saharan Africa. The Queen of Sheba from the Bible was supposedly descended from them and ruled part of their former land. They also managed to cross the Sahara and conquered part of ancient Egypt for a couple generations, before getting kicked out and the remains of their people left behind regelated to a menial labor underclass by the native Egyptians. So you can imagine why getting called that would be treated as fighting words.
It means "from Nubia", as in the region along the Nile to the south of the ancient Egyptian territories (modern-day southern Egypt/northern Sudan). The ancient Nubians and Egyptians were... not pals. They spent most of the last three millennia BC alternately conquering and enslaving each other.
Nubia was the name of a protohistorical kingdom or region in East Africa, famed among contemporary North African and non-African people they had contact with for having really good-looking women compared to the rest of Sub-Saharan Africa. The Queen of Sheba from the Bible was supposedly descended from them and ruled part of their former land. They also managed to cross the Sahara and conquered part of ancient Egypt for a couple generations, before getting kicked out and the remains of their people left behind regelated to a menial labor underclass by the native Egyptians. So you can imagine why getting called that would be treated as fighting words.
Little more complicated than that. Nubia/Kush was a major political rival of Egypt basically all throughout its existence, and archaeological evidence shows that while Egyptian political rhetoric might relegate their southern neighbors to a hated enemy who was only worth enslaving, there was definitely a lot of cultural exchange going on between the two. Also they were also famous for their incredible archers, and notably the Nubian kingdoms (queendoms?) stayed independent powers well past Egypt and got a reputation for pushing back the Roman legions on several occasions. Lastly, I think the Ethiopian church has Opinions on the idea that the Queen of Sheba was a Kushite.