Quick answer
Douglas is a boy name meaning "dark river".
Meaning and history
From a Scottish surname that was from the name of a town in Lanarkshire, itself named after a tributary of the River Clyde called the Douglas Water. It means "dark river", derived from Gaelic dubh "dark" and glais "water, river" (an archaic word related to glas "grey, green"). This was a Scottish Lowland clan, the leaders of which were powerful earls in the medieval period. The Gaelic form is Dùghlas or Dùbhghlas. It has been used as a given name since the 16th century.
Notes and context
Usage: scottish, english