This is a bit of American slang that we have taken a while to adopt. It dates back to the 1960s in the US where it seems particularly popular with speakers of Black American English, but there are just a few instances of it surfacing in Australian English. It has a couple of spellings and pronunciations. If you favour a pronunciation of bourgeois that goes [bore-jwah], then you probably adopt the shortened form bourgie, pronounced [bore=gee]. If you favour the pronunciation [boo=jwah], then you probably choose the spelling bougie pronounced [boo-jee].
The fact that it hasn’t settled yet is a mark of its newness.
Basically bougie is an epithet that you can throw at anyone you consider to be pretentious, particularly someone who is consciously displaying their upwardly mobile trajectory in society by choosing clothes, food, drinks, cars, etc. that are expensive and trendy.