seashore n.

Middle English se, seo, from Old English sæ,"sheet of water, sea, lake, pool," from Proto-Germanic *saiwa- (source also of Old Saxon seo, Old Frisian se, Middle Dutch see, Dutch zee, German See, Swedish sjö), of unknown origin, outside connections "wholly doubtful" [Buck], and an IE etymon "has generally been doubted" [Boutkan]. The meaning "any great mass or large quantity" (of anything) is from c. 1200.

Germanic languages also use the more general Indo-European word (represented by English mere (n.1)) but have no firm distinction between "sea" and "lake," either large or small, by inland or open, salt or fresh. This may reflect the Baltic geography where the languages are thought to have originated. The two words are used more or less interchangeably in Germanic, and exist in opposite senses (such as Gothic saiws "lake, marshland," marei "sea;" but Dutch zee "sea," meer "lake"). Compare also Old Norse sær "sea," but Danish sø, usually "lake" but "sea" in phrases. German See is "sea" (fem.) or "lake" (masc.).

Boutkan writes that the sea words in Germanic likely were originally "lake," and the older word for "sea" is represented by haff. The single Old English word sæ glosses Latin mare, aequor, pontus, pelagus, and marmor. The range in the Old English word included "the expanse of salt water that covers much of the world" to individual great, distinctly limited bodies of water; it also was used of inland seas, bogs, lakes, rivers, and the Bristol Channel.

The meaning "dark area of the moon's surface" is attested from 1660s (see mare (n.2)); before the invention of telescopes they were supposed to be water. The phrase sea change "transformation," literally "a change wrought by the sea," is attested from 1610, first in Shakespeare ("The Tempest," I.ii). Sea legs, humorous colloquial term implying ability to walk on a ship's deck when she is pitching or rolling is from 1712. At sea in the figurative sense of "perplexed" is attested from 1768, from literal sense (in reference to ships) of "out of sight of land" (c. 1300).

The sea, the most intact and ancient thing on the globe.

    Everything it touches is a ruin; everything it abandons is new.

[Paul Valéry, "Notebook" entry, 1921, transl. Nathaniel Brudavsky-Brody]

"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.

also sea-side, "the land bordering on the sea, the margin or brim of the sea," c. 1200, from sea + side (n.). Especially in England, "the seacoast as a resort for pleasure or health," 1782; as an adjective in this sense from 1781. The meaning "the side facing the sea" seems to be late (19c.) and rare.

Other Middle English "seaside, seashore" words included sees koste (mid-14c.), sewarth (Old English sæwaroþ, from wār "seashore, beach"), se-ground, se-brimme, sæ-strand, sea-half (Old English sæhealf), se-bank (mid-14c.). Old English used særima "sea-rim," sæ-strande, etc.