Etymology
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obelisk (n.)

"tapering rectangular stone column with a pyramidal apex," 1560s, from French obélisque (16c.) and directly from Latin obeliscus "obelisk, small spit," from Greek obeliskos "small spit, obelisk, leg of a compass," diminutive of obelos "a spit, pointed pillar, needle, broach; obelisk; bar of metal used as a coin or weight," a word of uncertain origin; according to Beekes, "clearly Pre-Greek." In printing, "a sign resembling a small dagger" (1580s). In dictionaries it is used to mark obsolete words. Greek obelos also was "a mark used in writing; horizontal line used as a diacritic." Related: Obeliscal; obeliskine.

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obol (n.)

ancient Greek small coin and weight, 1660s, from Latin obolus, from Greek obolos, the name of a coin (sixth part of a drachme); identical with obelos "a spit, needle, broach; bar of metal used as a coin or weight" (see obelisk). So called from the original shape. Middle English had obolus as the name of a small measure of weight, also ob "halfpenny," from Latin ob., abbreviation of obolus.

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pyramid (n.)

1550s "massive monumental stone structure of polygonl plan, the sides of which slope in planes to a common apex," also a geometrical solid resembling this, (earlier in Latin form piramis, late 14c., or nativized in Middle English as piram), from French pyramide (Old French piramide "obelisk, stela," 12c.), from Latin pyramides, plural of pyramis "one of the pyramids of Egypt," from Greek pyramis (plural pyramides) "a pyramid," which is apparently an alteration of Egyptian pimar "pyramid."

Greek pyramis also meant "kind of cake of roasted wheat-grains preserved in honey," and in this sense is said to derive from pyros "wheat" on the model of sesamis. According to some old sources the Egyptian pyramids were so called from their resemblance to the form of the cake, but Beekes points out that "the form of the cake is actually unknown."

Figurative of anything with a broad base and a small tip. Financial senses are by 1911. Related: Pyramidal (late 14c., piramidal).

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rock (n.1)

[stone, mass of mineral matter], Middle English rokke, roche "stone as a substance; large rocky formation, rocky height or outcrop, crag," from Old English rocc (as in stanrocc "stone rock or obelisk") and directly from Old North French roque, variant of Old French roche, which is cognate with Medieval Latin rocca (8c.), from Vulgar Latin *rocca, a word of uncertain origin. According to Klein and Century Dictionary, sometimes said to be from Celtic (compare Breton roch). Diez suggests Vulgar Latin *rupica, from Latin rupes "rocks."

In Middle English it seems to have been used principally for large rock formations but occasionally of individual boulders. The extended sense of "a stone of any size" is by 1793, American English colloquial, and long was considered incorrect.

It is an error to use rock for a stone so small that a man can handle it : only a fabulous person or a demi-god can lift a rock. [Century Dictionary]

The meaning "precious stone," especially a diamond, is by 1908, U.S. slang; the sense of "crystallized cocaine" is attested from 1973 in West Coast slang. Also used attributively in names of animals that frequent rocky habitats, as in rockfish, rock badger, rock lobster (the last attested by 1843).

Rock is used figuratively for "a sure foundation, something which gives one protection and security" (especially with reference to Christ), from the 1520s (Tyndale); but it also has been used since the 1520s as "cause or source of peril or destruction," an image from shipwrecks.

Between a rock and a hard place "beset by difficulties with no good alternatives" is attested by 1914 in U.S. Southwest:

to be between a rock and a hard place, vb. ph. To be bankrupt. Common in Arizona in recent panics; sporadic in California. [Dialect Notes, vol. v, part iv, 1921]
As an example of fine distinctions, a party of men were discussing the present situation of the German army, this week. One remarked that the Germans were between the devil and the deep sea; while another corrected him by saying that the Germans were between the upper and nether mill stone. The third man whose name is Pilgreen, and who works in the treasurer's office, simply remarked that the Germans were between a rock and a hard place. [local item in the Pouteau (Oklahoma) Weekly Sun,  Oct. 1, 1914]

The rock-scissors-paper game is attested by that name by 1976 (as paper stone and scissors by 1941). Sources agree it is based on Japanese Jan Ken Po or Jan Ken Pon (or Janken for short); the Japanese game is described in English publications by 1879.

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