I had a discussion with a friend that revolved around cultural nuance. We're from different cultures, so we had to explain a few things to each other. It got me to thinking about where I live. There's some prejudice down in the valley (Boulder) about my town. Outsiders would assume it's because our town appears "rich." When you take it at face value, yeah, you might assume that. But Boulder property values are nearly double ours and there's plenty of money in wee little Boulder. So that's not it, not entirely.
At its heart, it's about the actual construction and "ruining" natural habitat. See, Boulder is landlocked by open space and they always sort of considered my town to be part of that without it actually being part of it... so when houses went up 15 years ago Boulderites freaked that they were ruining prairie dog habitat, etc.
That's something you could only really know by living it.
So I got to thinking, what in Boulder County 2065 could you only know by living it? One thing is that calling Indigos stands for Independents, as well as the blue scarves they wear to keep the ash and dirt out of their lungs. Another is that calling Wiccans by that term is derogatory. Everyone in the culture knows that Wiccans refer to themselves as witches, but outsiders persist in using the other term as a slur. As for the Christians, even they disdain the revived practice of kissing their bishop's ring as well as the militarized church. But they know the backstory, the history, of how the church saved the day decades ago when the US ran out of resources and went haywire. They take all that for granted. I'm sure there are others, but I can't come up with one off the top of my head. These, though, are the things that bring a fictional world to life.
What is something in your WIP that your characters only know by living it?