My French guest holds out her right hand, and moves the four fingers up and down against the thumb (almost like a mouth opening and closing) to signify "You talk too much" or "I don't agree with what you're saying">

I've heard the above derivation of the "V" sign, as archers exulting that they still had two, before; but I don't believe a word of it. It sounds like an urban myth (or an everyday myth of county folk *BG*) to me. How long has the "V" sign been around, anyway? Is there evidence of its use before WW2?

Where I come from, it's use is known colloquially as "rodding".