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 Father of Turks was another historical figure who used the pose. Many attribute Ataturk's extraordinary charisma to simple gestures such as the hand-in-waistcoat pose. 
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.