What is "hand-in-waistcoat"

The hand-in-waistcoat (also referred to as hand-inside-vest, hand-in-jacket, hand-held-in, or hidden hand) was a gesture commonly found in portraiture during the 18th and 19th centuries. The pose appeared by the 1750s to indicate leadership in a calm and firm manner. The pose is most often associated with Napoleon I of France due to its use in several portraits made by his artist, Jacques-Louis David, amongst them the 1812 painting Napoleon in His Study. The pose, thought of as being stately, was copied by other portrait painters across Europe and America. The majority of paintings and photographs show the right hand inserted into the waistcoat/jacket but some sitters appear with the left hand inserted. The pose was also frequently seen in mid-nineteenth century photography.