I was looking into the history of the U.S. Dollar, and our first "official" dollar coin was the "Milled Spanish Dollar" (AKA Pieces of Eight) which was technically a Spanish Peso, a silver coin marked Eight Reales, minted in the Spanish colonies and widely available in the colonies thanks to piracy of the Spanish galleons transporting the coins to Spain.
When the U.S. "Dollar" was defined by law, they took a number of Spanish Milled Dollars in circulation, weighed them, and got the average weight, then defined a "Dollar" as containing the same quantity of silver. Milled Spanish Dollars remained legal tender, concurrent with U.S. Silver Dollars until the Coinage Act of 1857.
One Reale coins were called "bits." But One Reale coins were rare in the colonies because only Eight Reale coins were shipped to Spain, so you made change by cutting the Eight Reale coin in halves, quarters and eighths (bits). This is why a quarter is called "two bits" and it's also why the Spanish Milled Dollars were sometimes called "Pieces of Eight." Earlier Eight Reale coins were crudely hammer struck instead of milled and are the ones portrayed in pirates movies.
The reason they were called "Spanish Dollars" was because it was of similar size to an earlier coin used by the colonists, the Dutch Daalder, also known as the "Lion Dollar" which itself was called a Daalder because it was of similar size to the Austrian "thaler" which was a widespread standard for silver coins in Europe ("Daalder" is the Dutch translation of "Thaler", "Dollar" is the English translation of "Daalder")
Hammer Struck Peso (Pirate's Pieces of Eight)
Milled Peso (Spanish Milled Dollar or Pieces of Eight)
Daalder (Dutch Lion Dollar)
Austrian Thaler