An Etymological Dictionary of the German Language/Fließ
Fließ, Fließ, n., ‘fleece,’ from the equiv. MidHG. vlies, n.; comp. Du. vlies, AS. fleós, n., E. fleece; also a mutated form AS. flýs, flýss, MidHG. vlius, earlier ModHG. fleuss, flüss. A second parallel form is represented by ModHG. Flaus. In East Teut. the cognates are wanting; whether Goth. *fl- or *þliusis, n. (comp. fliehen), is to be assumed we cannot say, since satisfactory references to non-Teut. forms have not yet been produced. To explain Vließ from Lat. vellus is futile, since the latter is more probably primit. allied to Wolle, and to regard Vließ as borrowed from vellus is impossible; flechten, Flachs, &c., are also totally unconnected with the word.