"Whittling" the index finger and "wagging it" upright at someone are likely related gestures. The punishment in medieval England for petty thief (and by association, light crimes) was to have a finger lopped off. According to my favorite origin stories, pointing your finger at someone is to assign blame to them, and then you either making a chopping motion (England), sawing motion (Wales) or "lopping off toward them" motion with the other index finger, warning them, "You could lose a finger like that." --- The related gesture of wagging a raised index finger once you have their attention is a similar reminder they don't want to lose a finger. The first gesture is accompanied sometimes by the words, "shame on you"; the second, "no no no."