Etymology
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instability (n.)
early 15c., from Old French instabilité "inconstancy" (15c.) or directly from Latin instabilitatem (nominative instabilitas) "unsteadiness," from instabilis "unsteady, not firm, inconstant, fickle," from in- "not, opposite of" (see in- (1)) + stabilis (see stable (adj.)).
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inconstance (n.)

late 14c., inconstaunce, "changeableness in action, feeling, etc.; fickleness, unsteadiness," from Old French inconstance "inconstancy, instability" (13c.) and directly from Latin inconstantia "inconstancy, fickleness," abstract noun from inconstans "changeable, inconsistent" (see inconstant). In English, inconstancy is now the usual word.

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reflux (n.)

early 15c., "a flowing back" (of the sea, etc.), "ebb tide," also figurative of instability, from Medieval Latin refluxus, from Latin re- "back, again" (see re-) + fluxus "a flowing," from fluere "to flow" (see fluent). Digestive sense is recorded from 1937; reflux-valve is attested by 1853.

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sand (n.)
Origin and meaning of sand

"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), from PIE *bhs-amadho- (source also of Greek psammos "sand;" Latin sabulum "coarse sand," which is the source of Italian sabbia, French sable), suffixed form of root *bhes- "to rub."

Historically, the line between sand and gravel cannot be distinctly drawn. Used figuratively in Old English in reference to innumerability and instability. General Germanic, but not attested in Gothic, which used in this sense malma, related to Old High German melm "dust," the first element of the Swedish city name Malmö (the second element meaning "island"), and to Latin molere "to grind."

Metaphoric for innumerability since Old English. In compounds, often indicating "of the shore, found on sandy beaches." In old U.S. colloquial use, "grit, endurance, pluck" (1867), especially in have sand in (one's) craw. Sands "tract or region composed of sand," is by mid-15c.

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