A precursor to the modern suit waistcoat. From this page:
What is today recognized as a man’s three-piece suit began to develop in the late 17th century and was well established by the 18th century. In the early 17th century most men wore as the outer layer of garments a tailored doublet and full breeches. In the middle of that century the vest was introduced to European fashion from Asia Minor. Looser forms of doublets left unbuttoned allowed the long vest to be seen beneath.