E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Marked thus $, either scutum or 8, a dollar being a piece of eight [reals]. The two lines indicate a contraction, as in lb.
The word is a variant of thaler (Low German, dahler; Danish, daler), and means a valley, our dale. The counts of Schlick, at the close of the fifteenth century, extracted from the mines at Joachims thal (Joachims valley) silver which they coined into ounce-pieces. These pieces, called Joachims-thalers, gained such high repute that they became a standard coin. Other coins being made like them were called thalers only. The American dollar equals 100 cents, in English money a little more than four shillings.