Everybody knows it’s because he was a chronic masturbator.
Oh are silly replies ok now? I wanted to say it’s because he was making armpit farts.
The (probably apocryphal) story that I heard was that portrait painters would charge extra for hands. That theory falls down when you consider that, were that the case, a leader wishing to show his wealth would purposefully show his hands to demonstrate his wealth. Maybe throw some gang signs.
The traditional prestige pose was equestrian: mounted on horseback, your left hand holding the reins, your right hand holding a sword or a marshal’s baton. (Or, if you were royal, a scepter.)
For a non-equestrian portrait, this presents the problem of what to do with your hands. A scholar might be depicted holding a book, or a surgeon holding a scalpel. But if both hands are occupied, it makes the painting look too “busy”, and distracts the viewer’s attention from the your face.
Putting one hand in the waistcoat got it out of the way.
For the same reason that half the male portraits in my senior high school yearbook have the thumb and forefinger of one hand cradling the chin. That’s just the way it was done at the time.
I read somewhere it was supposed to look like he was reaching for a pistol, or something like that.
I read somewhere that that hand was deformed so he was hiding it. But I guess that’s bunk now.
So, what you’re saying is Napoleon went to Glamour Shots.
A previous thread on the subject: http://boards.straightdope.com/sdmb/showthread.php?p=7143022&highlight=hand+shirt+napoleon#post7143022