There has been much speculation and argument about the derivation of the name Sedgwick. Some have concluded that the name derives from the grass "sedge." Others say that it has nothing to do with sedge, as the grass is unknown in the area from which Sedgwicks emanated. I agree with the latter.
The name was apparently "Siggeswick" or "Seggeswick" at some early time. Adam Sedgwick, the famous geologist, asserted this in an oft-quoted letter, and stated that
"It is good German, and means the 'village of victory', probably designating some place of successful broil, where our rude Saxon or Danish ancestors first settled in the country and drove the old Celtic tribes out of it..." (Note that "Sieg Heil!" = "Hail to Victory.")
Regarding the "grass" theory, Adam Sedgwick had this to say.
"The word 'Sedge' is not known in the northern dialects of our island, and the plant itself does not exist among our valley, but a branch of our clan settled in the low, marshy regions of Lincolnshire, and seems to have first adopted the more modern spelling, and at the same time began to use a bundle of sedge ... as the family crest."
A 1600's map of Westmoreland shows the spelling as "Seggeswick." This confirms the "Siggeswick" or "Seggeswick".
Mike Siswick, webmaster of Siswick.org, has done an extensive study of the derivation of his name. (Siswick.org: Origin of Surname) He tells us that Sedgwick and Siswick are both variations of the Siggeswick name, that "Siggi" is an Old Norse pet name or nickname with no real meaning of its own, and that Siggeswick would be "The Village/Settlement of Siggi” a Scandinavian settler. Mike also says that "sig" is the Norse word for victory, so whether it's "Village of Victory" or "Siggi's Village" he believes "110%" that our ancestry is Norse.
I still feel that there is some possibility that our ancestors were Saxons or Angles. However, with Mike Siswick's extensive and recent, modern, research, I think it most likely that his conclusion is correct and that we are of Norse origin. Either way, after seeing the old spelling of the town, I certainly don't buy the grass bit!
Full text of Adam Sedgwick's letter.
Francis Morris Sedgwick agreed with Adam Sedgwick, and documented this in A Genealogy of the Sedgwick Family in America since 1635, page 1.
Hubert Merrill Sedgwick seems to have ultimately believed the "sedge" theory, as documented in New Haven Colony Historical Society MSS B46 Box 14 Folder L sheets 11 and 12. (Be sure to read both pages.) I think he would have swung the other way if he had seen John Speede's map.