dollar (n.)youdaoicibaDictYouDict[dollar 词源字典]
1550s, from Low German daler, from German taler (1530s, later thaler), abbreviation of Joachimstaler, literally "(gulden) of Joachimstal," coin minted 1519 from silver from mine opened 1516 near Joachimstal, town in Erzgebirge Mountains in northwest Bohemia. German Tal is cognate with English dale.

The thaler was a large silver coin of varying value in the German states (and a unit of the German monetary union of 1857-73 equal to three marks); it also served as a currency unit in Denmark and Sweden. English colonists in America used the word in reference to Spanish pieces of eight. Due to extensive trade with the Spanish Indies and the proximity of Spanish colonies along the Gulf Coast, the Spanish dollar was probably the coin most familiar in the American colonies and the closest thing to a standard in all of them; it was used in the government's records of public debt and expenditures; it had the added advantage of not being British. The Continental Congress in 1786 adopted dollar as a unit when it set up the modern U.S. currency system, which was based on the suggestion of Gouverneur Morris (1782) as modified by Thomas Jefferson. None were circulated until 1794 .
When William M. Evarts was Secretary of State he accompanied Lord Coleridge on an excursion to Mount Vernon. Coleridge remarked that he had heard it said that Washington, standing on the lawn, could throw a dollar clear across the Potomac. Mr. Evarts explained that a dollar would go further in those days than now. [Walsh]
Phrase dollars to doughnuts attested from 1890; dollar diplomacy is from 1910. The dollar sign ($) is said to derive from the image of the Pillars of Hercules, stamped with a scroll, on the Spanish piece of eight. However, according to the Bureau of Engraving and Printing of the U.S. Department of the Treasury:
[T]he most widely accepted explanation is that the symbol is the result of evolution, independently in different places, of the Mexican or Spanish "P's" for pesos, or piastres, or pieces of eight. The theory, derived from a study of old manuscripts, is that the "S" gradually came to be written over the "P," developing a close equivalent of the "$" mark. It was widely used before the adoption of the United States dollar in 1785.
[dollar etymology, dollar origin, 英语词源]
eight (n.)youdaoicibaDictYouDict
late 14c., eighte, earlier ehte (c. 1200), from Old English eahta, æhta, from Proto-Germanic *akhto (cognates: Old Saxon ahto, Old Frisian ahta, Old Norse atta, Swedish åtta, Dutch acht, Old High German Ahto, German acht, Gothic ahtau), from PIE *okto(u) "eight" (cognates: Sanskrit astau, Avestan ashta, Greek okto, Latin octo, Old Irish ocht-n, Breton eiz, Old Church Slavonic osmi, Lithuanian aštuoni). From the Latin word come Italian otto, Spanish ocho, Old French oit, Modern French huit.

For spelling, see fight (v.). Meaning "eight-man crew of a rowing boat" is from 1847. The Spanish piece of eight (1690s) was so called because it was worth eight reals (see piece (n.)). Figure (of) eight as the shape of a race course, etc., attested from c. 1600. To be behind the eight ball "in trouble" (1932) is a metaphor from shooting pool. Eight hours as the ideal length of a fair working day is recorded by 1845.
piaster (n.)youdaoicibaDictYouDict
also piastre, 1620s, "Spanish dollar, piece of eight," also used as the name of a monetary unit and coin of Turkey (1610s, in Turkish called ghurush, but originally debased Spanish dollars), from French piastre, from Italian piastra "thin metal plate," short for impiastro "plaster," from Latin emplastrum, from Greek emplastron (see plaster). The Italian word was applied to the Spanish silver peso, later to the Turkish coin based on it. Compare shinplaster.
piece (n.)youdaoicibaDictYouDict
c. 1200, "fixed amount, measure, portion," from Old French piece "piece, bit portion; item; coin" (12c.), from Vulgar Latin *pettia, probably from Gaulish *pettsi (compare Welsh peth "thing," Breton pez "piece, a little"), perhaps from an Old Celtic base *kwezd-i-, from PIE root *kwezd- "a part, piece" (cognates: Russian chast' "part"). Related: Pieces.

Sense of "portable firearm" first recorded 1580s; that of "chessman" is from 1560s. Meaning "person regarded as a sex object" is first recorded 1785 (compare piece of ass, human beings colloquially called piece of flesh from 1590s; also compare Latin scortum "bimbo, anyone available for a price," literally "skin"). Meaning "a portion of a distance" is from 1610s; that of "literary composition" dates from 1530s. Piece of (one's) mind is from 1570s. Piece of work "remarkable person" echoes Hamlet. Piece as "a coin" is attested in English from 1570s, hence Piece of eight, old name for the Spanish dollar (c. 1600) of the value of 8 reals.
PIECE. A wench. A damned good or bad piece; a girl who is more or less active and skilful in the amorous congress. Hence the (Cambridge) toast, may we never have a PIECE (peace) that will injure the constitution. ["Dictionary of Buckish Slang, University Wit and Pickpocket Eloquence," London, 1811]
two bits (n.)youdaoicibaDictYouDict
"quarter dollar," 1730, in reference to the Mexican real, a large coin that was divided into eight bits; see bit (n.1). Compare piece of eight (see piece (n.)). Two bits thus would have equaled a quarter of the coin. Hence two-bit (adj.) "cheap, tawdry," first recorded 1929.