Source : Webster's Revised Unabridged Dictionary (1913)

Halibut \Hal"i*but\ (?;277), n. [OE. hali holy + but, butte,
   flounder; akin to D. bot, G. butte; cf. D. heilbot, G.
   heilbutt. So named as being eaten on holidays. See {Holy},
   {Holiday}.] (Zo["o]l.)
   A large, northern, marine flatfish ({Hippoglossus vulgaris}),
   of the family {Pleuronectid[ae]}. It often grows very large,
   weighing more than three hundred pounds. It is an important
   food fish. [Written also {holibut}.]

Source : WordNet®

halibut
     n 1: lean flesh of very large flatfish of Atlantic or Pacific
     2: marine food fish of the northern Atlantic or northern
        Pacific; the largest flatfish and one of the largest
        teleost fishes [syn: {holibut}]