According to legend, French soldier Nicolas Chauvin was badly wounded in the Napoleonic Wars and received a meager pension for his injuries. After Napoleon abdicated, Chauvin maintained his fanatical Bonapartist belief in the messianic mission of Imperial France, despite the unpopularity of this view under the Bourbon Restoration. His single-minded devotion to his cause, despite neglect by his faction and harassment by its enemies, started the use of the term.[2]

https://en.m.wikipedia.org/wiki/Chauvinism