The obvious place to look for the origin of fish names is in another Icon of England, the O.E.D. Let's begin with cod. The dictionary says, "Origin uncertain: the name is known only as English." It goes on to suggest a link with another old meaning of cod - "bag". In his book Cod, Mark Kurlansky also considers this question:

"In Middle English, cod mean "a bag or sack", or by inference, "a scrotum", which is why the outrageous purse that sixteenth-century men wore at their crotch... was called a codpiece. Samuel Johnson's 1655 dictionary defines cod as "any case or husk in which seeds are lodged." Does this have anything to do with the fish? Most scholars doubt it but offer no other solution for the origin of the name."

The fish could have been known as a "bag fish" from its appearance. But does a cod really look like a bag?

"Origin uncertain"

The phrase "origin uncertain" appears again and again in the O.E.D.'s fish etymologies. Mackerel is derived from an Old French word, "makarel : of origin uncertain." Herring is "origin uncertain", though it has been conjectured that it was linked with a German word, heri (host), as if "the fish that comes in hosts". The O.E.D. suggests this and then immediately dismisses the idea, for the name was originally pronounced "heering".

Occasionally the O.E.D. adds a tentative suggestion to "origin uncertain". Haddock is "origin uncertain. The suffix "-ock" appears to be a diminuitive, as in bullock, dunnock, hillock". So a haddock was a little had, whatever that might be…

Plaice and sole

Thankfully, there are a few fish with descriptive names whose origin is not uncertain. Plaice comes by way of Old French plais from the Latin plattus, meaning "flat" (as in "plate"). Sole is an Old French word meaning "sandal" from the Latin solea (hence the "sole of a shoe"). So a plaice is flat like a plate, while a sole resembles a sandal!

Halibut, gurnard and huss

The O.E.D. derives halibut from "holy" and "butt" (another name for a flat fish). So a halibut is a holy flat fish "supposed to be so called from being so commonly eaten on holy days" - the religious festivals when eating meat was banned.

One of the most striking etymologies is for gurnard, which is derived from an Old French word meaning "grunter". According to the illustrative quotation, from Daniel Defoe, it was given this name "from its grunting noise when taken".

The huss was originally called a "husk".  According to the O.E.D., its skin "was much used by fletchers for smoothing and polishing arrows". And so it may have been called a "husk" (skin) fish.

If these word origins are correct, they show that fish have been named in a variety of ways. The gurnard could only have been named by the fishermen who heard it grunt, while the huss would have been named by the fletchers who used its skin. The halibut was probably named by the ordinary people, who ate it on holy days.