The ancestry sites like to say that Ellis became a popular baby name after the Crusades1. Something about the prophet Elijah, a tribute to the Holy Land. Since people were named by who their father was, it became a surname. That might be the case.
They also like to say Ellis is a Welsh name. The double ll in Welsh names is pronounced the same way as the tl in Quetzalcoatl2, so Ellis probably isn’t an Anglicized Welsh word
There’s an excellent site in Glasgow that states the name came from the Saxon King Ælle3, who invaded the East of England in the 5th century AD. Another great baby name. That would make Ellis an Anglo-Saxon name.
1 The Crusades, 
in case you missed that day in history class, were a series of twelve Christian forays into the Middle East to wrest the Holy Land from the Muslim infidels. The Crusades were all complete failures, which is why so many of us objected to President Bush’s use of the word in describing his Big Plan for revenge after 9-11. The Crusaders were absolved of their sins if they died in battle, which may have been a contributing factor. Gunpowder didn’t reach Europe until after the 9th Crusade, another contributing factor.
2 To make the sound, make the t sound like a cross between a hard c and a t without a syllable break before the l. This should keep you occupied for hours. The English pronounce it as “kl” and leave it at that. A good description of how to form the sound is available on the Gwybodiadur site.
3 Type Æ as alt-0-1-9-8 on a Microsoft Windows compatible keyboard. Code it with the Æ or Æ html entities. This is why the ligature Æ is falling into disuse.
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