A slang term usually aimed at American Democrat voters (generally those who are upper middle class or rich) who care more about being mildly inconvenienced by not being able to go to brunch than they do about people who aren't as better off as them.
The term originated during the Women's March on Washington in 2017, where several white women held up signs that said "If Hillary were president, we'd be at brunch."
"In American politics you can be one of three people: The Millionaire, The poor Republican/Democrat, or the Brunch Democrat. No one likes the Brunch Democrat."
"BGB" for short, this is a phenomenon which occurs generally on Sundays but may happen on a Saturday. It starts with the idea of cooking brunch, but balloons out of control with a mini-party of anywhere between 5-15 people in the kitchen cooking, blasting music and dancing. The actual food itself becomes a distraction as it's more important to be in the dance cage at this point drinking straight from the champagne bottle that was originally meant for classy mimosas. Most people call out of work the next day.