halibut

noun, plural (especially collectively) hal·i·but, (especially referring to two or more kinds or species) hal·i·buts.

1. either of two large flatfishes, Hippoglossus hippoglossus, of the North Atlantic, or H. stenolepis, of the North Pacific, used for food.
2. any of various other similar flatfishes.

noun plural -buts or -but

1. the largest flatfish: a dark green North Atlantic species, Hippoglossus hippoglossus, that is a very important food fish: family Pleuronectidae
2. any of several similar and related flatfishes, such as Reinhardtius hippoglossoides (Greenland halibut)

n.

large flatfish, early 15c., perhaps from hali “holy” (see holy) + butte “flatfish;” supposedly so called from its being eaten on holy days (cf. cognate Dutch heilbot, Low German heilbutt, Swedish helgeflundra, Danish helleflynder). For second element see butt (n.4).