sand-castle (n.)

late Old English castel "village" (this sense from a biblical usage in Vulgar Latin); later "large building or series of connected buildings fortified for defense, fortress, stronghold" (late Old English), in this sense from Old North French castel (Old French chastel, 12c.; Modern French château), from Latin castellum "a castle, fort, citadel, stronghold; fortified village," diminutive of castrum "fort," from Proto-Italic *kastro- "part, share;" cognate with Old Irish cather, Welsh caer "town" (probably related to castrare via notion of "cut off," from PIE root *kes- "to cut"). In early bibles, castle was used to translate Greek kome "village."

Latin castrum in its plural castra was used for "military encampment, military post" and thus it came into Old English as ceaster and formed the -caster and -chester in place names. Spanish alcazar "castle" is from Arabic al-qasr, from Latin castrum.

Castles in Spain "visionary project, vague imagination of possible wealth" translates 14c. French chastel en Espaigne (the imaginary castles sometimes stood in Brie, Asia, or Albania) and probably reflects the hopes of landless knights to establish themselves abroad. The statement that an (English) man's home is his castle is from 16c.

THAT the house of every man is to him as his castle and fortress, as well for his defence against injuries and violence, as for his repose .... [Edward Coke, "Semaynes Case," 1604]

"water-worn detritus finer than gravel; fine particles of rocks (largely crystalline rocks, especially quartz); the material of the beach, desert, or sea-bed;" Old English sand, from Proto-Germanic *sandam (source also of Old Norse sandr, Old Frisian sond, Middle Dutch sant, Dutch zand, German Sand), cognate with Greek psammos "sand;" Latin sabulum "coarse sand" (which is the source of Italian sabbia, French sable). This was said to be from a suffixed form of a PIE root *bhes- "to rub," but de Vaan says the Latin is from a substrate word and Beekes suggests for the origin of psammos "Pre-Greek *sam- 'sand, mud'."

Historically, the line between sand and gravel was not distinct. A general Germanic word but it is not attested in Gothic, which used in this sense malma, related to Old High German melm "dust" and the first element of the Swedish city name Malmö (the second element meaning "island"), and to Latin molere "to grind."

Sand has been a figure of innumerability or instability since Old English. In compounds, often it indicates "of the shore, found on sandy beaches." The old U.S. colloquial sense of "grit, endurance, pluck" is by 1867, especially in have sand in (one's) craw. Sands "tract or region composed of sand," is by mid-15c.