An Etymological Dictionary of the German Language/Annotated/Fließ
Fließ, Fließ, neuter, ‘fleece,’ from the equivalent Middle High German vlies, neuter; compare Dutch vlies, Anglo-Saxon fleós, neuter, English fleece; also a mutated form Anglo-Saxon flýs, flýss, Middle High German vlius, earlier Modern High German fleuss, flüss. A second parallel form is represented by Modern High German Flaus. In East Teutonic the cognates are wanting; whether Gothic *fl- or *þliusis, neuter (compare fliehen), is to be assumed we cannot say, since satisfactory references to non-Teutonic forms have not yet been produced. To explain Vließ from Latin vellus is futile, since the latter is more probably primitively allied to Wolle, and to regard Vließ as borrowed from vellus is impossible; flechten, Flachs, &c., are also totally unconnected with the word.