Etymology Dictionary

HALIBUT,  a large, flat-fish.  (E.)   'Hallibut, a fish like a plaice;' Kersey's Dict., ed. 1715.   Cotgrave translates O.F. flatelet by 'a hallibut (fish).'   Compounded of M.E. hali, holy (see Holy), and butte, a flounder, plaice, which occurs in Havelok, 759.   So called because excellent eating for holidays; the sense being 'holy (i.e. holiday) plaice.'   The fish often attains to a large size, and weighs as much as 400 lbs.   The cognate languages have similar names for it. + Du. heilbot; from heilig, holy, and bot, a plaice.   Cf. Swed. helgflundra, from helg, holidays, and flyndra, a flounder:  Dan. helle-flynder, from hellig, holy, and flynder, a flounder. [†]

ADDENDA

It is suggested that the M.E. butte is rather 'flounder' than 'plaice;' cf. G. butte, a flounder.   The Tauchnitz Du. Dict. gives Du. bot, 'a flounder, plaice.'   The fact is simply that fish-names, like plant-names, are in a state of great confusion.