Etymology
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underground (adv.)
1570s, "below the surface," from under + ground (n.). As an adjective, attested from c. 1600; figurative sense of "hidden, secret" is attested from 1630s; adjectival meaning "subculture" is from 1953, from adjectival use in reference to World War II resistance movements against German occupation, on analogy of the dominant culture and the Nazis. Noun sense of "underground railway" is from 1887 (shortened from phrase underground railway, itself attested from 1834).
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economy (n.)
Origin and meaning of economy

1530s, "household management," from Latin oeconomia (source of French économie, Spanish economia, German Ökonomie, etc.), from Greek oikonomia "household management, thrift," from oikonomos "manager, steward," from oikos "house, abode, dwelling" (cognate with Latin vicus "district," vicinus "near;" Old English wic "dwelling, village," from PIE root *weik- (1) "clan") + nomos "managing," from nemein "manage" (from PIE root *nem- "assign, allot; take"). Meaning "frugality, judicious use of resources" is from 1660s. The sense of "wealth and resources of a country" (short for political economy) is from 1650s.

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economy (adj.)
1821 as a term in advertising, at first meant simply "cheaper," then "bigger and thus cheaper per unit or amount" (1950). See economy (n.).
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Underground Railroad (n.)
"network of U.S. anti-slavery activists helping runaways elude capture," attested from 1847, but said to date from 1831 and to have been coined in jest by bewildered trackers after their slaves vanished without a trace. Originally mostly the term for escape networks in the (then) western states of the U.S.
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economist (n.)

1580s, "household manager," from French économiste; see economy + -ist. Meaning "student of political economy" is from 1804.

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overground (adj.)
"situated above ground" (as opposed to underground), 1879, from over- + ground (n.).
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econometric (adj.)
1933, from economy + -metric. Related: Econometrics.
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cistern (n.)

"natural or artificial receptacle for holding water or some other fluid," mid-13c., from Old French cisterne "cistern; dungeon, underground prison" (12c., Modern French citerne), from Latin cisterna "underground reservoir for water," from cista "chest, box," from Greek kistē "box, chest" (see chest). Related: Cisternal.

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subway (n.)
1825, "underground passage" (for water pipes or pedestrians, later for electrical wires), from sub- + way (n.). The sense of "underground railway in a city" is first recorded 1892, in reference to London.
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undercroft (n.)
"crypt of a church; underground vault," late 14c., from under + croft.
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