Not an official historian but my podcast History Lessons with Caleb, Mike & Terry did a show on the various cultures of Ancient Mesopotamia. In my research I saw that the original word for king started as a word that just meant big man. I think that chieftains who's tribal village became the earliest city states would become king of that city. Then Sargon came, conquered a bunch of them and became the first emperor but I think he was referred to as a king of kings.