What is a parvenu? I saw the word in William Makepeace Thackeray's book Vanity Fair.

A parvenu is a person who has suddenly acquired wealth or power, especially one who isn't fully accepted socially by the class associated with the higher position. You might be more familiar with the similar term nouveau riche, which refers to someone who has only recently become rich and often connotes tasteless ostentation and lack of culture.

In Vanity Fair, social-climbing Rebecca Sharp makes an impression in Paris:

It has been mentioned that Rebecca, soon after her arrival in Paris, took a very smart and leading position in the society of that capital, and was welcomed at some of the most distinguished houses of the restored French nobility. The English men of fashion in Paris courted her, too, to the disgust of the ladies their wives, who could not bear the parvenu.