seashell (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]

"hard outer covering," Middle English shel, shelle, from Old English sciell, scill, Anglian scell "seashell; eggshell," which is related to Old English scealu "shell, husk," from Proto-Germanic *skaljo "piece cut off; shell; scale" (source also of West Frisian skyl "peel, rind," Middle Low German schelle "pod, rind, egg shell," Gothic skalja "tile"), with the shared notion of "covering that splits off," from PIE root *skel- (1) "to cut." Italian scaglia "chip" is from Germanic.

Also in late Old English as "a coating or layer." The general sense of "protective outer covering of some invertebrates" is in Middle English (by c. 1400 as "house of a snail;" by 1540s in reference to a tortoise or turtle); the meaning "outer layer of a nut" (or a fruit considered as a nut) is by mid-14c. With notion of "mere exterior," hence "empty or hollow thing" by 1650s. The meaning "hollow framework" is from 1791; that of "structure for a band or orchestra" is attested from 1938. To be out of (one's) shell "emerged into life" is by 1550s.

Military use for "explosive projectile" is by 1640s, first of hand grenades, and originally in reference to the metal case in which the gunpowder and shot were mixed; the notion is of a "hollow object" filled with explosives. Hence shell shock, "traumatic reaction to the stress of battle," recorded by 1915.

Shell game "a swindle" is from 1890, from a version of the three-card game played with a pea and walnut shells.