Merriam-Webster Collegiate Dictionary - halibut

 noun  (plural ~; also ~s)  Etymology: Middle English halybutte, from haly, holy holy + butte flatfish, from Middle Dutch or Middle Low German but; from its being eaten on holy days  Date: 14th century any of several marine flatfishes (especially Hippoglossus hippoglossus of the Atlantic and H. stenolepis of the Pacific) that are widely used for food and include some of the largest bony fishes