I am trying to get my brother into English and Irish traditional music (with some success on the Irish side). He asked me a really interesting question which had never occurred to me. He said, "Why do so many songs have whack-fol-de-day etc.?" I said my guess is that in years gone by songs were usually sung unaccompanied, and perhaps it was a way of imitating an instrument. I know, for instance, that this is exactly what a lot of mouth music is/was: diddling for dances where there were no instruments. Can one safely assume that the same is true of the odd bit of nonsensical diddling in an otherwise lyrical song? Any scholars out there?