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early 15c., from Old French jetee "a jetty, a projecting part of a building," also "a throw," noun use of fem. past participle of jeter "to throw" (see jet (v.)). Notion is of a structure "thrown out" past what surrounds it.
any of a variety of engineering structures connected with river, harbour, and coastal works designed to influence the current or tide or to protect a harbour or beach from waves (breakwater). The two principal kinds of jetties are those constructed at river mouths and other coastal entrances and those used for the berthing of ships in harbours and offshore where harbour facilities are not available. Jettylike structures may be built out at intervals from the banks of rivers where a wide channel must be narrowed to concentrate the current and thus help maintain a navigable channel. These structures-variously termed spurs, spur dikes, and groins-may also be projected from the concave side of a river to retard bank erosion.