swiftness n. + Add to list US //ˈswɪftnəs// UK //swˈɪftnəs// swift·ness SynonymsEtymology Synonyms fastnessspeedOriginOEswiftnes→MEswiftnesse→ENswiftness From Middle English swiftnesse, swyftnesse, from Old English swiftnes (“swiftness”), equivalent to swift + -ness.