dollar (n.) Look up dollar at Dictionary.com
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. Continental Congress July 6, 1785, adopted dollar when it set up U.S. currency, on suggestion of Gouverneur Morris and Thomas Jefferson, because the term was widely known but not British. But 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.