Napoleon, n. — Green’s Dictionary of Slang

Napoleon n.

[the clichéd image of the mad believing that they are Napoleon or some other world figure]

(US black) a madman, an eccentric.

1944		‘Jiver’s Bible’ in D. Burley Orig. Hbk of Harlem Jive.
1957		R. Prather Always Leave ’Em Dying 36: She paused for a moment and spoke with two white-jacketed men, whom I assumed were either doctors or a couple of Napoleons.