An Etymological Dictionary of the German Language/flau
flau, adj., ‘feeble, stagnant, insipid, dull,’ simply ModHG.; borrowed in the last century from LG. flau, Du. flauw, ‘languid, faint, indifferent,’ which, with E. flew, ‘soft, tender,’ are derived from Rom. Considering the late appearance of the cognates, and the area to which they are confined, it is certain that they originated in Fr. flou, OFr. flau, floi; the latter is of Teut. origin (see lau), so that ModHG. flau is finally derived from a pre-Teut. hléwa-. Comp. Flanke.