This is a cape, or a jacket, inspired by a so called casaque. That was a garment with six buttoned openings: in front, in back and on each side of the arms. It can be buttoned to have sleeves, or to be a cape - or, like here, to have one sleeve and (on the other side) have place for a bound-up arm.
Here it is made of vadmal (see
, and has buttons made of old coins.