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 strike oil in their backwoods 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."