upstart
n.n. a person who tries to be more important or powerful than they really are. People use this word when someone is acting like they belong to a higher social class than they actually do.
n. a person who pretends to be more important or wealthy than they actually are. Often carries a derogatory or dismissive tone, implying a lack of legitimate social status or merit.
He is a total upstart who thinks he knows everything.
The established family members were annoyed by the upstart who tried to take over the business without experience.
The novel explores the tension between the traditional aristocracy and the upstart bourgeoisie, whose sudden wealth and influence threaten the established social order.
From Middle English upstarten, upsterten, equivalent to up- + start.