Entries linking to coast guard
early 14c., "margin of the land;" 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," and PIE root *ost-), but de Vaan dismisses this and calls it "an isolated word without etymology."
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 the English verb meaning "a slide or sled down a snowy or icy hillside," first attested 1775 in American English. Expression the coast is clear (16c.) is an image of landing on a shore unguarded by enemies; to clear the coast (1520s) was to make it suitable for landing.
early 15c., "one who keeps watch, a body of soldiers," also "care, custody, guardianship," and the name of a part of a piece of armor, from French garde "guardian, warden, keeper; watching, keeping, custody," from Old French garder "to keep, maintain, preserve, protect" (see guard (v.)). Abstract or collective sense of "a keeping, a custody" (as in bodyguard) also is from early 15c. Sword-play and fisticuffs sense is from 1590s; hence to be on guard (1640s) or off (one's) guard (1680s). As a football position, from 1889. Guard-rail attested from 1860, originally on railroad tracks and running beside the rail on the outside; the guide-rail running between the rails.