The hand-in-waistcoat pose was the practice of placing one hand inside the top garment in order to convey calm assurance and elevated character.
This gesture dates all the way back to classical times in the 6th century B.C., when Greek circles often disregarded as rude the act of speaking with both hands outside of tunics, especially when conducting political activities.
"And so decorous were those public men of old, Pericles, Themistocles, and Aristeides, that to speak with the arm outside the cloak, as we all do nowadays as a matter of course, was regarded then as an ill-mannered thing, and they carefully refrained from doing it. And I can point to a piece of evidence which seems to me very weighty and tangible. I am sure you have all sailed over to Salamis, and have seen the statue of Solon there. You can therefore yourselves bear witness that in the statue that is set up in the Salaminian market-place Solon stands with his arm inside his cloak. Now this is a reminiscence, fellow citizens, and an imitation of the posture of Solon, showing his customary bearing as he used to address the people of Athens."
But by the time Aeschines came to life, the practice had already been dismissed and was only revived many centuries later when painters of the 18th and 19th century started adopting it relentlessly. It first appeared in Godfry Kneller’s Self-Portrait (1710), Richardson's Horace Walpole (1734-35) and Thomas Gainsborough's Self-portrait (1727-1788).
The fad only reached its peak when several portraits of Napoleon Bonaparte (1769-1821) painted by celebrated artists like Jean-Auguste-Dominique Ingres, Jacques-Louis David and Hippolyte Delaroche started circulating. It was then that the hand-in-waistcoat gesture reverberated through European and American portraiture.
By the end of the 19th century, the fad started to daze out for the most part, although it was still occasionally adopted in 20th century photography among which is found the famous shot of Joseph Stalin taken in 1948.
(all pictures are courtesy of wikimedia)