English word halibut comes from Middle English *halibutt (A halibut.. Fishes similar to the halibut.).

Halibut etymology history^?

Halibut detailed word origin explanation

Dictionary entry	Language	Definition
*halibutt	Middle English (enm)	A halibut.. Fishes similar to the halibut.
halibut	English (eng)	A large flatfish of the genus Hippoglossus, which sometimes leaves the ocean floor and swims vertically.