An Etymological Dictionary of the German Language/flattern

by Friedrich Kluge

flattern, vb., ‘to flutter, dangle,’ in MidHG. vladern from MidHG. vlëdern (see Fledermaus); MidDu. flatteren, E. to flatter, akin to flutter, also MidE. fliteren, E. to flitter; AS. floterian, MidE. floteren, ‘to undulate,’ are, however, certainly allied to the root flut, ‘to flow.’