Origins
The word 'coast' arrives in English from Old French 'coste' (rib, side, flank; shore, coast), from Latin 'costa' (rib, side, wall).βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ The metaphor is anatomical: just as a rib is the side of the body, the coast is the side of the land β the flank where the continent faces the sea. This body-to-landscape metaphor is characteristic of how Latin geographical vocabulary developed, treating the earth as a body with sides, limbs, and extremities.
Latin 'costa' had both literal and figurative uses. Literally, it meant a rib β one of the curved bones of the thorax. Figuratively, it meant any side or flank: the side of a hill, the wall of a building, the edge of a territory. The geographical sense (the side of a land-mass facing the sea) passed into all the Romance languages: French 'cΓ΄te' (coast, rib, hillside), Spanish 'costa' (coast), Italian 'costa' (coast, slope), Portuguese 'costa' (coast). The CΓ΄te d'Azur is the 'Azure Side,' Costa Rica is the 'Rich Side/Coast,' and the Ivory Coast is the 'cΓ΄te' where ivory was traded.
The word entered English after the Norman Conquest, arriving with the wave of French vocabulary that reshaped the language in the 12th through 14th centuries. In English, the rib-sense was eventually lost (we use 'rib' from Old English 'ribb,' a native Germanic word), and 'coast' was specialized to the geographical meaning. However, the anatomical connection survives in medical terminology: 'intercostal' (between the ribs), 'costal' (of or relating to the ribs), and 'subcostal' (beneath the ribs) all use the Latin form directly.
French Influence
Several surprising English words are relatives. 'Accost' (to approach someone boldly, to confront) comes from Old French 'acoster' (to come alongside, to draw up flank-to-flank), from Latin 'ad-' (to) + 'costa' (side). To accost someone is etymologically to come up to their side β a naval metaphor of one ship drawing alongside another. 'Cutlet' comes from French 'cΓ΄telette,' the diminutive of 'cΓ΄te' (rib) β a cutlet is literally 'a little rib.' 'Cuesta' (a geological term for a hill with one steep side and one gentle slope) is the Spanish reflex of the same Latin word.
The verb 'to coast' (to move without effort, to glide) developed from the nautical sense of 'sailing along the coast' β that is, following the shoreline rather than striking out across open water. Coastal sailing was easier and safer than ocean crossings, requiring less effort and skill. By extension, 'to coast' came to mean any movement that proceeds with minimal effort: coasting downhill on a bicycle, coasting through an exam. The compound 'roller coaster' (1888) combines this 'effortless gliding' sense with the hills and valleys of the coastal landscape.
The legal distinction between 'coast' and 'shore' has exercised English-speaking lawyers for centuries. In general legal usage, 'coast' refers to the broader region near the sea (including land above the high-water mark), while 'shore' refers specifically to the intertidal zone. This distinction matters for jurisdictional questions: coastal law, maritime law, and property law each govern different strips of the water's edge.