"low, hollow place, often boggy," 1580s, special use of Scottish swaill "low, hollow place," or East Anglian dialectal swale "shady place" (mid-15c.); both probably from Old Norse svalr "cool," from Proto-Germanic *swalaz. A local word in England, in U.S. given broad application, especially to the lower tracts of the prairie and recently to landscaping features in suburban developments.
