What is "sumorsaete"

The Sumorsaete were an Anglo-Saxon group living in what is now Somerset, presumably around the town of Somerton. They are evidently the source of the county's name. The group may have been established as early as the 577, when the Saxons conquered part of the area from the Britons, but they are not mentioned by name until 845. They may have been related to the obscure Glastening or Glestinga, who may be the source of the name Glastonbury.