From Britain or land of the Britons
Britanie is a modern phonetic spelling variant of Brittany. It originally served as a geographic surname for people from the Brittany region of France, which was settled by Britons fleeing the Anglo-Saxon invasions of England.
Less common today
Britanie isn't in the latest US Top 1000. It last peaked at #3,089 in 1991.
US births per 5-year period, 1990βpresent (SSA data).