Fitchet, or fitch is the fur of the polecat. It has a yellow ground, with long, soft, black shining hairs on its surface, which are exclusively used for artists' brushes. The fur is not in great demand for fabrics as it emits an unpleasant odour which is difficult to dissipate. Found on http://www.probertencyclopaedia.com/browse/AF.HTM
A fitchet is a vertical opening at the hip of a dress or surcoat so as to allow the wearer to put their hand through and onto something underneath, perhaps a pocket in an undergarment. Found on http://www.probertencyclopaedia.com/browse/PF.HTM