Well, no one knows, really. What people agree on is that Memphis, Tennessee was named after Memphis, Egypt.
The Egyptian Memphis is now just ruins, in the southern part of Cairo. It was long-abandoned in 1819, when the Tennessee city was founded. But, during it’s heydey in the millennia BC, it was one of the largest cities in the world, and the capital of Egypt for several hundred years.
Memphis, Tennessee was named by its three founders: John Overton, James Winchester and Andrew Jackson.
Wikipedia states, clearly:
They named it after the ancient capital of Egypt on the Nile River
It cites a 1945 book called “Names on the Land : A Historical Account of Place-Naming in the United States” which I could not find access to.
A site about Memphis history states this:
The trio named the city after the capital of ancient Egypt. Though their exact line of reasoning is uncertain, it’s possible that they thought the nearby Mississippi River evoked the mighty Nile and envisioned America’s Memphis as a hub for culture, transit and trade.
Memphis magazine agrees with that reasoning this:
The Tennessee city was named for its relation to the river.
The name “Memphis” is from the Egyptian “mennefer” which means “his beauty” or “perpetual beauty.”
Memphis is mentioned eight times in the Old Testament, all in the books of the prophets. For example, Jeremiah 46:19 foretells of it’s destruction:
O thou daughter that dwellest in Egypt, furnish thyself to go into captivity; for Memphis shall become a desolation, and shall be burnt up, without inhabitant.