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.
Slovenian, Croatian, Czech, and Polish: nickname from kos ‘blackbird’, denoting someone thought to resemble the bird in some way, for example someone with a good singing voice. This is a very common surname in Slovenia and Croatia.