alongshore (adj.)

Middle English, from Old English andlang "entire, continuous; extended" (adj.); also "alongside of" (prep.); from and- "opposite, against" (from Proto-Germanic *andi-, *anda-, from PIE *anti "against," locative singular of root *ant- "front, forehead") + lang "long" (see long (adj.)).

Reinforced by its Old Norse cognate endlang. The prepositional sense was extended in Old English to "through the whole length of." Of position, "lengthwise," from c. 1200; of movement, "onward," from c. 1300. The meaning "in company, together" is from 1580s. All along "throughout" is attested from 1690s.

"land bordering a large body of water," c. 1300, from Old English scora, sceor- (in place-names) or from Middle Low German schor "shore, coast, headland," or Middle Dutch scorre "land washed by the sea," all probably from Proto-Germanic *skur-o- "cut," from PIE root *sker- (1) "to cut."

This is the usual theory, "but the etymological notion is not easy to determine" [OED]. It has been proposed as meaning "division" between land and water, but if the word began on the North Sea coast of the continent, it might as well have meant originally "land 'cut off' from the mainland by tidal marshes" (compare Old Norse skerg "an isolated rock in the sea," related to sker "to cut, shear").

Old English words for "coast, shore" were strand (n.), waroþ, ofer. Few Indo-European languages have such a single comprehensive word for "land bordering water" (Homer uses one word for sandy beaches, another for rocky headlands).

General application to "country near a seacoast" is attested from 1610s. In law, typically the tract between the high- and low-water marks (1620s). Shore-bird is attested from 1670s; the sailor's shore-leave by 1845.