Middle English halybutte combined holy with a flatfish word. The name refers to eating it on holy days.
Butte came from Middle Dutch or Middle Low German; the alternatives are not consecutive stages.
Sources
- Merriam-Webster: halibut
Middle English haly and butte mean holy and flatfish; the name refers to holy-day consumption and the fish element has alternative donor languages.
- Online Etymology Dictionary: holy
Also built on *holy*: holiday, Halloween