Why is Napoleon depicted with his hand in his shirt? I think I’ve seen pictures of others striking a similar pose.
It was a quite common portrait pose of the period. It lent stature and posture to the posee, and gave him something to do with his hands. Or at least one of them.
Note the similar pose by The Duke of Wellington.
I reckon they were fiddling with their nipples, the dirty little munchkins.
Hey, you have to entertain yourself somehow while they’re carving away. 
Yes, i’m aware he wouldn’t be there for the carving.
Here’s a pretty lengthy answer:From The Napoleon Series
It’s in the FAQ, but here’s a bit:
One answer I read in a discussion of American Civil War portraits, where the subject invariably had his hand inside his jacket/shirt, was that American artists suffered from a cultural cringe and slavishly imitated European portrait painters and their paintings of Napoleon. They didn’t know that the reason for the pose was that Napoleon suffered from ulcers and carried his hand on his stomach to alleviate the constant pain. Americans didn’t know this and painted their portraits that way because that was how it was supposed to be done. There probably is no truth to the assertions but I love the ideas that generations of portrait painters posed their subjects as they did because Napoleon had a tummy ache.
I always thought he had an itch.
Yes, the “Napoleon” pose was popular with American military officers, well past the Civil War. I have a portrait of Gen. Beauregard in this pose. The US Army though pretty higly of French militaria in that era-our soldiers wore french-style kepis, unforms, etc. We even had zouaves (those funky baggy pants and all)-all were french imports.