flunder

15th century (East Prussian), also vluder, from Middle Low German vlundere, from Old Saxon *flunthria, eventually from Proto-Germanic *flunþrijǭ. The High German word must be from Low German given the marine context. However, Low German attestations are equally late and there are some indications that the word was not originally widely known. It may have been regionally restricted, or otherwise a borrowing from Old Norse flyðra and/or Middle English flounder.