It contains the element branda- 'borderland' (referring to Buckland being on the eastern edge of the Shire), but also evokes Branda-nîn, 'border-water' (the early hobbitish name for the Brandywine river).
The river Branda-nîn was later jokingly altered to Bralda-hîm, 'heady ale'. According to Tolkien, "only a very bold hobbit would have ventured to call the Master of Buckland braldagamba in his hearing". 
Note on translation
While "Marchbuck" would be a literal translation of the name, it would lose the traditional connection with Brandywine, apparent in the actual Brandagamba-Branda-nîn. Tolkien rather translated the name as "Brandybuck" in order to retain this connection.
- buck at Wiktionary.