I searched out this article today after reading an entry in a mythology encyclopedia about a "witches' chair" used to extract "confessions" from prisoners during witch trials in the 17th Century in Bamberg, Germany. The chair was "...made of iron...covered with studs and heated over a fire." (Stuart Gordon, The Encyclopedia of Myths and Legends, pp. 736-737. Headline Book Publishing, 1993)
Perhaps the expression could have earlier roots than those posited in the sources cited here.