shellfish

The word 'shellfish' is a transparent Modern English compound, directly combining two native Old English elements. 'Shell' originates from the Proto-Germanic \skaljō, meaning a division or a shell, which itself derives from the Proto-Indo-European root \skel-, meaning to cut or split, reflecting the concept of a hard, separable covering. 'Fish' comes from the Proto-Germanic \fiskaz, from the PIE root \peysk-, meaning fish. The compound logic is straightforward: an aquatic creature defined by its hard external shell. Over time, its application broadened from a simple descriptive term to a standard culinary and biological category encompassing crustaceans and molluscs, despite their taxonomic differences from true fish.

Structure: shell (hard outer covering) + fish (aquatic animal) - shell: From Old English sciell (shell, eggshell), related to Old Norse skel. Its functional role is to denote the defining physical characteristic. - fish: From Old English fisc (fish). Its functional role is to broadly classify the creature as an aquatic animal, though not a true fish in the scientific sense.