somebody who has risen suddenly, e.g. from a low position to wealth or power, esp somebody who behaves with unwarranted arrogance or self-importance

(used before a noun) denoting, relating to, or characteristic of such a person

a little upstart, vulgar being, with her Mr. E., and her caro sposo … and all her airs of pert pretension and underbred finery — Jane Austen