Cod
Middle English: of unknown origin; one suggestion is that the word is the same as Old English cod(d) ‘bag’, because of the fish's appearance.
wiktionary
From Middle English cod, codde, from Old English cod, codd(“bag, pouch”), from Proto-Germanic *kuddô, from Proto-Indo-European *gewt-(“pouch, sack”), from *gew-(“to bend, bow, arch, vault, curve”). Cognate with Scots cod, codd, coad, kod(“pillow, cushion”), Low German Koden, Kon(“belly, paunch”), Middle Dutch codde(“scrotum”), Danish kodde(“testicle”), Swedish kudde(“cushion”), Faroese koddi(“pillow”), Icelandic koddi(“pillow”).
From Middle English cod, codde, of uncertain origin:
Origin unknown. Attested in reference to a person (though not always a stupid or foolish person) from the end of the 17th century. The Oxford English Dictionary (1891) notes that a suggested link to codger is unlikely, as cod appears much earlier.
etymonline
cod (n.)
large sea fish, edible and widely distributed in colder seas, mid-14c. (late 13c. in a surname, Thomas cotfich), of unknown origin; despite similarity of form it has no conclusive connection to the widespread Germanic word for "bag" (represented by Old English codd, preserved in cod-piece). Codfish is from 1560s. Cod-liver oil, known at least since 1610s, was recommended medicinally from 1783 but did not become popular as a remedy until after 1825.
