The hand-in-waistcoat was a gesture commonly found in men's portraiture during the 18th and 19th centuries. Napoleon I of France was most well known for the gesture and is readily associated with this gesture because of the several portraits made by his artist, Jacques-Louis David. Theories state the gesture was done by Napoleon because of a stomach pain he had, but the pose was common in portraits from the mid-18th century. Along with this, many people state he had an itchy skin disease, or possible even breast cancer.
The pose traces back to classical times — Aeschines, founder of a rhetoric school, suggested that speaking with an arm outside one's toga was bad manners.