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.)).

place-name element in Afghanistan, Pakistan, etc., from Persian -stan "country," from Indo-Iranian *stanam "place," literally "where one stands," from PIE *sta-no-, suffixed form of root *sta- "to stand, make or be firm."

used figuratively for "generic suburban tract housing," American English, from the vast planned real estate developments built by the firm Levitt & Sons Inc., the first on Long Island, 1946-51 (more than 17,000 homes), the second north of Philadelphia (1951-55).

also clampdown, 1940 in the figurative sense "a firm, oppressive or harsh suppression or preventive action," from verbal phrase clamp down "use pressure to keep down" (1924). The verbal phrase in the figurative sense is recorded from 1941. See clamp (v.) + down (adv.).

early 15c., from Old French interstice (14c.) and directly from Latin interstitium "interval," literally "space between," from inter "between" (see inter-) + stem of stare "to stand" (from PIE root *sta- "to stand, make or be firm"). Related: Interstices.

1590s, "not firm or secure in position," from un- (1) "not" + steady (adj.). Similar formation in Old Frisian unstadich, German unstätig, Middle Dutch onstadich. Meaning "marked by irregularity" is from 1680s. Related: Unsteadily (1550s).

"unsteady, unstable, taking no fixed position," 1827, with -ic + Greek astatos "unstable, not steadfast," from a- "not, without" (see a- (3)) + statos "placed, standing" (from PIE root *sta- "to stand, make or be firm"). Related: Astatically.

"time, moment" (archaic), from Old English stund "point of time, time, hour," cognate with Old Saxon stonda, Old Frisian stunde, Dutch stondi, German Stunde "hour," from PIE root *sta- "to stand, make or be firm."

c. 1300, "essential nature, real or essential part," from Old French sustance, substance "goods, possessions; nature, composition" (12c.), from Latin substantia "being, essence, material," from substans, present participle of substare "stand firm, stand or be under, be present," from sub "up to, under" (see sub-) + stare "to stand," from PIE root *sta- "to stand, make or be firm."

Latin substantia translates Greek ousia "that which is one's own, one's substance or property; the being, essence, or nature of anything." Meaning "any kind of corporeal matter" is first attested mid-14c. Sense of "the matter of a study, discourse, etc." first recorded late 14c.

late 14c., "not empty or hollow, hardened;" of figures or bodies, "having three dimensions," from Old French solide "firm, dense, compact," from Latin solidus "firm, whole, undivided, entire," figuratively "sound, trustworthy, genuine," from suffixed form of PIE root *sol- "whole."

The meaning "firm, hard, compact" is from 1530s. Of arguments, etc., "substantial" (opposed to frivolous or flimsy). The meaning "entirely of the same stuff" is from 1710. Of qualities, "well-established, considerable" c. 1600. Of food from c. 1700.

As a mere intensifier, "thoroughly, downright," by 1830. The slang sense of "wonderful, remarkable" is attested by 1920 among jazz musicians.

As an adverb, "solidly, completely," 1650s. Solid South in U.S. political history is attested from 1858 on the notion of unanimity in voting; solid in this sense (in reference to New York) is by 1855. Solid state as a term in physics is recorded from 1953; the meaning "employing printed circuits and solid transistors" (as opposed to wires and vacuum tubes) is from 1959.