Probably from sea and wall, a structure of stone or other materials intended for a defense or security against the sea. This name, though seemingly local, may have various significations; suil, in the Gaelic, is a willow; suail, small, inconsiderable. Su, south, and wold, wald, wild, well, an uncultivated place, a wood, a plain, a lawn, hills without wood: Suwold, Suwall, Suwell.
Arthur, William (1857) An Etymological Dictionary of Family and Christian Names. New York: Sheldon, Blakeman. Public Domain.