People who're into class-based identity politics use the word as a general pejorative and a way of 'other-izing' people they don't like. In my experience this is one of the most common uses of the word, and also the least useful.
The word originally (and still does) refer to a specific stratum of the middle class - the upper middle class (in today's terms) that own capital (material resources that have been invested and are being worked) - people like business owners, landlords, and investors.
What does it mean to be bourgeois in this sense? - you support state protection of private property, you (at least partly) live off of the labor of others, you have enough disposable income to be able to consume commodities with little or no utility (often cultural commodities) that differentiate yourself from other social classes (like buying private-box tickets at sporting events, art collecting, wearing boutique and high-fashion-branded clothing etc). Thorstein Veblen's descriptions of what he called 'the leisure class', and their 'conspicuous consumption' and 'conspicuous leisure' are very illustrative here.
The other components of what we think of as 'the middle class' today, such as 'professionals', homeowners, and the self-employed aren't strictly part of the bourgeoisie, because they don't own capital on a large scale. They do however have a stake in capitalism, and share certain cultural values with the bourgeoisie - they're pro-establishment, pro-property, and often share the same kind of moral delusions (eg. if your poor it's because you're lazy) and sense of propriety as the bourgeoisie. Because of these shared traits they're sometimes called 'petite (or petty) bourgeoisie'.
Now the interesting part (at least to me) is that these terms were coined when only a small portion of society self-identified as middle class. Today the portion of society that self-identifies as middle class is enormously inflated, mostly because the post-WW2 project of the welfare state drastically increased home-ownership, average wages/disposable income and living standards in general. That project was largely a response to the percieved existential threat that the USSR posed to capitalism - the welfare state was meant to give enough people a stake in capitalism to stave off the percieved threat of socialism. After all, men with mortgages and families to support don't go on strike.
There's much more to it of course, the large-scale abandonment of working class identity in favor of joining a newer and much more broadly defined 'middle class' is a very complex phenomenon, but my point is that 'middle class' basically means whatever you want it to now, and so it isn't that useful a concept for talking about class in an analytical (rather than identity-focused) way. 'Bourgeois' on the other hand is only used (in my experience) as a general insult or to describe a relatively strictly-defined group of people who are important to capitalism. Now that the idea that the middle class is shrinking has become a bit of a meme, the word 'bourgeoisie' is really useful when talking about the discrepancy between class conciousness and socio-economic reality.
