An Etymological Dictionary of the German Language/Annotated/flattern
flattern, verb, ‘to flutter, dangle,’ in Middle High German vladern from Middle High German vlëdern (see Fledermaus); Middle Dutch flatteren, English to flatter, akin to flutter, also Middle English fliteren, English to flitter; Anglo-Saxon floterian, Middle English floteren, ‘to undulate,’ are, however, certainly allied to the root flut, ‘to flow.’