The word "crayfish" is a textbook example of folk etymology — the process by which speakers reshape unfamiliar words to make them seem more logical. The original Middle English form was crevis, borrowed in the 14th century from Anglo-French crevice. This French word came from Old French crevice (also escrevisse), which was itself borrowed from a Germanic source: Old High German krebiz (crab, crayfish), from Proto-Germanic *krabitaz.
The transformation from crevis to "crayfish" occurred because English speakers, encountering a water-dwelling creature with a name that ended in an unfamiliar syllable, replaced that ending with the familiar word "fish." The creature lives in water, it is caught and eaten — calling it a fish made intuitive sense, even though crayfish are crustaceans, not fish. This folk-etymological reshaping was complete by the 16th century.
The Germanic source word *krabitaz has been extraordinarily productive. It gave German Krebs, which means both "crayfish" and "cancer" — the disease was named by Hippocrates, who used the Greek word karkinos (crab) because he thought the swollen veins around a tumour resembled crab legs. Latin borrowed the metaphor as cancer (crab), and German calqued it as Krebs. The zodiac sign Cancer is also a crab, creating
The French form écrevisse, which replaced the older crevice, developed through its own phonological changes. Swedish kräfta, Dutch kreeft, and Norwegian kreps all preserve the Germanic root. English "crab" itself comes from a related form, though its precise relationship to *krabitaz is debated.
Crayfish have enormous cultural significance worldwide. Louisiana's Cajun cuisine revolves around crawfish (the American dialectal form) — crawfish boils, étouffée, and bisque are defining dishes. The Swedish kräftskiva (crayfish party) is a beloved late-summer tradition involving paper hats, aquavit, and communal outdoor feasting. In Australia, "crayfish" refers to spiny lobsters, not the freshwater species — a usage that causes considerable confusion between Australian and Northern Hemisphere English
The dialectal variation in English is notable: "crayfish" is the standard British form, "crawfish" dominates in the American South, and "crawdad" is common in parts of the Midwest and Appalachia. The Australian use of "cray" as a casual abbreviation has recently collided with the unrelated slang adjective "cray" (short for crazy), creating an unintentional double meaning that amuses etymologists.