Linguistics
The song was originally composed and sung in Scots. It then made its way into mainstream English, but retains its Scottish flavour. Words like birk (for birch), lass and bonnie are typically Scots as are words like brae (hill) and braw (splendid). As is typical of such cases, quite a few of the less familiar words degenerated into nonsense words as the song travelled over cultures, the most interesting ones probably being Ethanside for Ythanside (banks of the River Ythan), and brasselgeicht for braes o' Gight (hills of Gight).
Read more about this topic: The Bonnie Lass O' Fyvie