The Collaborative International Dictionary of English v.0.48:

Halibut \Hal"i*but\ (h[o^]l"[i^]*b[u^]t; 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.] (Zool.)
   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.]
   [1913 Webster]