A parvenu is an upstart, somebody who's suddenly rich but doesn't fit into his new social status. If you're a parvenu, people might also describe you as "nouveau-riche" or an "arriviste." Maybe it's not quite so insulting in French.
The Beverly Hillbillies, a sitcom from the 1960s, featured the Clampetts, classic parvenus, who struck oil on their backwoods West Virginia swampland. They arrive at their mansion in Beverly Hills, dressed in overalls, in their pick-up truck, with their shabby furniture strapped on top. As parvenus, they don’t fit in — to say the least. Parvenu is from French, and it's the past participle of parvenir, "arrived."
