coastyoudaoicibaDictYouDict[coast 词源字典]
coast: [13] Latin costa meant ‘rib’ (hence the English medical term intercostal ‘between the ribs’), but also more generally ‘flank, side’. It was in this sense that it passed into Old French as coste, and subsequently into English. The modern meaning ‘seashore’ (which had already developed in Old French) arises from the shore being thought of as the ‘side’ or ‘edge’ of the land (compare seaside).

Amongst the senses of the French word little represented in English is ‘hillside, slope’; it was however adopted in North America for a ‘slope down which one slides on a sledge’, and came to be used in the mid 19th century as a verb meaning ‘sledge down such a slope’. That was the source of the modern verbal sense ‘freewheel’. The coster of costermonger [16] was originally costard, a variety of apple named from its prominent ‘ribs’.

And another hidden relative is cutlet [18], borrowed from French côtelette, literally ‘little rib’.
=> costermonger, cutlet, intercostal[coast etymology, coast origin, 英语词源]
coast (n.)youdaoicibaDictYouDict
"margin of the land," early 14c.; earlier "rib as a part of the body" (early 12c.), from Old French coste "rib, side, flank; slope, incline;" later "coast, shore" (12c., Modern French côte), from Latin costa "a rib," perhaps related to a root word for "bone" (compare Old Church Slavonic kosti "bone," also see osseous).

Latin costa developed a secondary sense in Medieval Latin of "the shore," via notion of the "side" of the land, as well as "side of a hill," and this passed into Romanic (Italian costa "coast, side," Spanish cuesta "slope," costa "coast"), but only in the Germanic languages that borrowed it is it fully specialized in this sense (Dutch kust, Swedish kust, German Küste, Danish kyst). French also used this word for "hillside, slope," which led to verb meaning "sled downhill," first attested 1775 in American English. Expression the coast is clear (16c.) is an image of landing on a shore unguarded by enemies.
coast (v.)youdaoicibaDictYouDict
late 14c., "to skirt, to go around the sides, to go along the border" of something (as a ship does the coastline), from Anglo-French costien, from the French source of coast (n.). The meaning "sled downhill," first attested 1775 in American English, is a separate borrowing. Of motor vehicles, "to move without thrust from the engine," by 1925; figurative use, of persons, "not to exert oneself," by 1934. Related: Coasted; coasting.