"The name Nile is derived from the Greek Neilos (Latin: Nilus), which probably originated from the Semitic root na?al, meaning a valley or a river valley and hence, by an extension of the meaning, a river. The fact that the Nile—unlike other great rivers known to them—flowed from the south northward and was in flood at the warmest time of the year was an unsolved mystery to the ancient Egyptians and Greeks. The ancient Egyptians called the river Ar or Aur (Coptic: Iaro), “Black,” in allusion to the colour of the sediments carried by the river when it is in flood. Nile mud is black enough to have given the land itself its oldest name, Kem or Kemi, which also means “black” and signifies darkness. In the Odyssey, the epic poem written by the Greek poet Homer (7th century BCE), Aigyptos is the name of the Nile (masculine) as well as the country of Egypt (feminine) through which it flows.
From the Encyclopedia Britannica.