Posing with a hand tucked into one's waistcoat or vest was considered a sign of good breeding for men in Napoleon's era. The pose is associated with Napoleon because of the famous painting "Napoleon in His Study."
The most famous painting of Napoleon in this pose, "Napoleon in His Study" by Jacques-Louis David, was not painted for Napoleon himself but was commissioned by Alexander Douglas, a Scottish nobleman who admired Napoleon. Napoleon did not even sit for this portrait. David painted his likeness from memory. Napoleon is also pictured in this pose in several other works by David.
The hand-in-waistcoat pose was often used by men of high standing in England in the 18th century. It was so well-known as to have become a cliche. Francois Nivelon's 1738 book, "A Book of Genteel Behavior," stated that the pose evoked "manly boldness tempered with modesty." The pose dates back to ancient Greece where it was a stance for orators recommended by Aeschines, the founder of a rhetoric school. He claimed that it was rude to speak with an arm outside of one's toga. Theories have stated that Napoleon kept his hand inside of his waistcoat because of stomach pain, breast cancer, a disfigured hand or skin disease. However, none of these have historical validity.