The only story behind this one that I have ever heard is that it is supposed to imitate the shaving down of a switch, the implication being that the switch would then be used to impose corporal punishment on the offending party. I do not know if this is accurate or not.

Ooh, interesting idea. I've never seen anyone do it in real life, and in fact I think I've only ever seen it in Mad Magazine cartoons.

Body language and gestures are definitely part of language. However, not everyone you ask will recognise them as such. To test the theory, put your hands in your pockets tomorrow and try to talk sensibly all day without taking them out.
Try a reputable dictionary of body language.

That must be a family idiom, or a regionalism. Here in my family in California, the "shamer" makes a fist, extends and points the index finger at the shamee, and shakes the finger up and down near the shamee's nose.

(I have seen the "shaving" gesture done in fun elsewhere).

I have used this many times over the years, index finger brushing against other index finger...I never knew where this expression came from;just wanted to know, just curious

I have seen both gestures, but, Liz, I would ask the same question. What does the "shaking the single index finger" gesture mean then? Is that supposed to be using the switch that was fashioned with the other gesture?:)

"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."