An Etymological Dictionary of the German Language/Annotated/Fledermaus
Fledermaus, feminine, ‘bat,’ from the equivalent Middle High German vlëdermûs, Old High German flëdarmûs, feminine; corresponds to Dutch vledermuis; English flittermouse does not occur in Anglo-Saxon, and may be due to the influence of Middle European Teutonic. That the animal was thought to be a mouse is shown by Anglo-Saxon hreape-, hrêremûs; the English term bat, Middle English backe, Danish aftenbakke (aften, ‘evening’), is unique. Fledermaus, literally ‘fluttering mouse,’ from Old High German flëdarôn, Middle High German vlëdern, ‘to flutter.’