From Britain or Brittany
A modern phonetic variant of Brittany, referring to the region in northwest France or the island of Great Britain. The name gained popularity in the late 20th century as a geographic surname-turned-given name. This specific spelling is a rare creative variation of the traditional forms.
Less common today
Brittainy isn't in the latest US Top 1000. It last peaked at #2,520 in 1990.
US births per 5-year period, 1990βpresent (SSA data).