Anglicized form of the Gaelic name Donnchadh, also used among people without Scottish connections. This was the name of a 7th-century Scottish saint (abbot of Iona) and two medieval kings of Scotland. The final ‐n in the Anglicized form seems to be the result of confusion with the Gaelic word ceann ‘head’ in the Latinized form Duncanus. Compare Irish Donagh.
French (Huguenot): from Old French osier ‘osier’, ‘willow’, with the preposition d(e) ‘from’, applied as a topographic name for someone who lived near a plantation of willows, or perhaps as a metonymic occupational name for a basket maker.