In both cases the old name was just a mispronunciation of the real name. That's right, some traveler heard Myanmar and repeated it as Burma . The name was later applied to the county where that river flows. He was a miner and journalist in the American West, and he apparently liked the sound of Calaveras . However, pirates have nothing to do with the place name.
Prayer too comes from Latin prex via an only slightly different route.
Some interesting related words which descended from precari are deprecate and precarious . Precarious entered English in the 17th century. This word was daler in 1533, having come from Low German and being a form of German Taler or Thaler . Taler was short for the lengthier Joachimstaler , a term applied to a 16th century coin. The coins were first minted in 1519.