Does anyone know where the “for shame” or “shame on you” hand motion came from? For those of you mystified by the question, the motion is done by pointing an index finger at the person who should be ashamed, and running the other index finger down the length of the pointer finger perpendicularly.
It’s a common expression, and I’m sure everyone has seen it. Anyone know its origins?
It’s just a minimised mime of slapping a childs wrist/fingers for doing something or touching something they shouldn’t.
Sounds reasonable. Do you, by any chance, have a link or other support?
Where I come from, in the 12-step culture, we call this motion the “Al-Anon Salute”. 
Robin
This is just an educated guess but…
The “for shame” movement seems to emulate two sticks rubbing together as if to start a fire. It may be that this was somehow linked to witchcraft trials in which the motion translates as something you should be burned for doing.
Believe it or not, this has been asked before. http://boards.straightdope.com/sdmb/showthread.php?threadid=35497
Of course, there was only the OP and one answer.
I’ve heard it simulates sharpening knives. The motion is obviously the same. I can’t remember what sharpening knives is supposed to do with it, though.
Johnny L.A.,
Is there no such thing as an original OP anymore? That’s nucking futz!
Thanks for the replies, BTW. The quest for knowledge continues.
Actually, that question was asked before. Let me find the link. 
I would like to know its origin, too. It always seemed like a whittling motion to me.
No, no references sorry, it’s just based on the fact that here the mimed ‘slap on the wrist’ motion is almost exclusively used for exactly the same purpose. You very occasionally see the ‘slap-finger’ thing you’ve described and everyone still knows exactly what it’s meant to be. Logical conclusion: one is derived from the other! Since I can see an obvious derivation of the ‘slap-on-the-wrist’ motion in use with parents every day I assume it’s the older gesture, and the other is minimised.
The term ‘slap-on-the-wrist’ as a sign of reproach for minor offenses is a very common expression (Do a Web search and I’m sure you’ll get thousand of matches).