One who speaks above his level of knowledge, experience or expertise. From Latin Ultra=above & beyond + crepis-idis=sole,keel.
The story goes that when the famous Greek painter Apellis made a full-body painting of Alexander the Great and showed it to the audience, a shoemaker approached him and made some comments about the rendering of the sandals of the great warrior. The painter promptly acknowledged the error and with a stroke of his brush corrected it. But after a while the same man started to comment about the robe of Alexander and the painter cut him short by saying, "Stick to the sandals, my valuable friend".