In the middle ages a number of cities and states in central Europe started minting large silver coins. For geological reasons the silver mines tended to be located in valleys. The words in old German for valley are Tal or Thal, which sounds roughly like "tahl" in English. The coins were soon identified by the name of the valley they came from, e.g. "Joachimsthaler" or "from Joachim's valley". The generic name for any large silver coins was shortened to Thaler (or Taler) which eventually morphed into "dollar" in English-speaking countries.
Sounds strange, but you can look it up (you did try Google, Wikipedia, etc. before posting, right?)