Anglicized form of Gaelic Domhnall. The final -d of the Anglicized form derives partly from misinterpretation by English speakers of the Gaelic pronunciation, and partly from association with Germanic-origin names such as Ronald. This name is strongly associated with clan Macdonald, the clan of the medieval Lords of the Isles, but is now also widely used by families with no Scottish connections.
Scottish: habitational name from any of the various places called Douglas from their situation on a river named with Gaelic dubh ‘dark’, ‘black’ + glas ‘stream’ (a derivative of glas ‘blue’). There are several localities in Scotland and Ireland so named, but the one from which the surname is derived in most if not all cases is 20 miles south of Glasgow, the original stronghold of the influential Douglas family and their retainers.