Origin: 1350–1400; Middle English halybutte, equivalent to haly (variant of holy) + butte flat fish (< MD); so called because eaten on holy days. Compare Dutch heilbot,German Heilbutt
1396, from hali "holy" + butte "flatfish;" supposedly so called from its being eaten on holy days (cf. cognate Du. heilbot, Low Ger. heilbutt, Swed. helgeflundra, Dan. helleflynder). The second element is a general Gmc. name applied to various kinds of flat fishes; cf. O.Swed. but "flatfish," M.E. butt