From the experts:
Brittany first arrived on the US popularity list in 1971, and rapidly zoomed up the charts, in the Top 100 a decade later. By 1986 it had entered the Top 10, becoming the third most popular girls’ name in the country by 1989. After such immense popularity, there has been a steep decline, but it remains a name evocative of one of the most beautiful and culturally interesting areas of France -- and much preferable to the contracted Britney. Brittany evolved as a modern coinage from the ancient French duchy Bretagne. Celtic Bretons emigrated from France to become the Bretons of English; later the name Britain came to signify the country.