It's not a widely accepted explanation, but I like it: "Mores" (pronounced moorase) is a Latin word meaning (more or less) "a custom". In mediaeval times, Latin was still used by the few people who could write: mainly churchmen. Old customs such as dancing, plays etc. may have been called "Mores" and this could easily have evolved, via "Morrys" etc. to Morris. Lots of Latin words have crept into general use (etcetera, vice versa, via) and some have been misheard and are now used in mainstream English in a distorted form.
Unless I missed it earlier in the thread, no one has mentioned the surviving traditions of Derbyshire, from the villages of Hayfield and Winster. There are also isolated survivals like the Britannia Coco-Nut Dancers from Bacup.
In all cases, Morris dancing is a traditional dance form from England, and from the border area between England and Wales. Teams (sides) were amde up of men from a particular village, and each village that had a side had its own distinctive style, and its own repertoire, ranging from one or two dances to 20 or more.
Morris dancing all but died out in the late 19th Century, and was then artificially revived. It is now primarily danced as a hobby, by teams founded in the 20th century. All or most of the "accepted wisdom" about it being a fertility ritual (etc. etc. blah blah) is sentimental nonsense, but those of us who do the dancing still value it highly for what it is - whatever it actually is!