An Etymological Dictionary of the German Language/Annotated/flau
flau, adjective, ‘feeble, stagnant, insipid, dull,’ simply Modern High German; borrowed in the last century from Low German flau, Dutch flauw, ‘languid, faint, indifferent,’ which, with English flew, ‘soft, tender,’ are derived from Romance. Considering the late appearance of the cognates, and the area to which they are confined, it is certain that they originated in French flou, Old French flau, floi; the latter is of Teutonic origin (see lau), so that Modern High German flau is finally derived from a pre-Teutonic hléwa-. Compare Flanke.