This is a weird family of fish... They're all born with their eyes on both sides of their head, but one of them starts migrating right away to the same side as the other eye. This way, they can lay flat on the bottom, with both eyes on the top side. You may think that the old toast "Here's mud in your eye" came from these fish, but I kinda doubt it. All the flatfish I've spoken to claim that monacles are cheaper than glasses with two lenses, so they all moved their eyes for cheaper lenses.

Anyway....on to the origin of the word "halibut". Originally in Old English, all the flatfish (including skates, plaice, turbots, and flounders) were all referred to as "butts". The largest and most highly praised of these was a species which reached 300 to 400 lbs and 8 feet long. That was the one they only ate on holy days. Because the old spelling for "holy" was "hali", their name for this fish was the "haly butt". It's pretty easy to sea how, over the centuries, haly butt became halibut.