Whiskey in the jar

Hello!
I've been learning to play this song www.thebards.net/music/lyrics/Whiskey_In_The_Jar.shtml  and the thing that keeps me puzzled is the refrain: what does "whack for the daddy ol' " mean? The 'old daddy' is clear, but the 'whack' leaves me in the woods... I've looked it up of course but it makes no sense. And, in your opinion, whose words are these? Does the protagonist pronounce them, or is he quoting his mistress Jenny?
Thanks!