"definitely 'a wound full sore'."
If it hasn't been mentioned. this is aa reference to the brother castrating himself.
The beauty of this ballad is its symbolism.

Mother I have tint (spoilt) my knife,
I loved it better than my life,
But I have tint a better thing,
The bonnie sheath my knife was in.

Powerful sexual symbolism or what!!
I Anglicised this many years ago - unfortunately sacrificing some of the lovely Scots words in the process.
I've just returned to it after many years and now find myself choking up on that beautiful verse - in this case, absence definitely has made the heart grow fonder.
Jim Carroll