Hello. I'm interested in the origin of this expression. There was a discussion on stack exchange http://english.stackexchange.com/q/128139/129806 about this. One user posited the idea that this expression came from Roman times when people shook forearms to make sure there were no weapons up their sleeves. Sounds at first glance to be BS but I'm not so sure.
We attribute this expression to magicians. Is this true, or did magicians lift this from the world of gamblers who might have gotten their expression from an older source such as Roman times? Did magicians first say this, or was someone making fun of a magician and said this?
A reference to a book, periodical, or credible internet source would be appreciated.