Cool
What does "cool" mean? Yes, we're actually going to analyze this.
(I was born in 1982, so my use of slang here is almost certainly dated. I'm not attempting to be "cool" linguistically! Map the ideas below to the terms you use for them.)
I'd say that at a basic level it refers to being nonchalant or unbothered.
But more typically it's a value judgment: the speaker is expressing that they feel positively about what they are calling "cool."
But what about when someone—say, a sterotypical school bully (which is sadly all too realistic)—says that something someone else likes is "not cool"? Are they just saying that their view is different, and agreeing to disagree? No, they're saying that there is an objective "cool," and what the other person likes doesn't qualify.
But the idea of an objective "cool" that we have to adhere to conflicts with another sense of the word "cool:" not caring what other people think of you. If you're concerned with only liking the things that are considered objectively "cool," and criticizing the things that aren't, that's maximally caring what other people think about you—so isn't that the opposite of "cool"?
If you want to adhere to the objective "cool," you might adopt interests, behavior, and appearance that fall under the objective "cool." But if you do, you might be accused of being a "poser" or "fronting." This might mean one of two things. First of all, you might be accused of trying to come across as something you're not. For example, I'm a suburban white guy. If I got really into hip hop music and fashion you could say that I'm trying to act like I'm from the hood but I'm not. Or if I got really into skater fashion you could say that I'm trying to act like I can skate, but I can't. Or, this accusation might be made even if there's no pretense on the person's end. For example, say it's the 90s and I get into alternative rock. Other kids who are into alternative rock might call me a poser—but does that mean I don't actually like the music? No. They're just communicating that they have a clique and I'm not accepted into it, regardless of my actual interests.
You know, hypothetically.
Recently in media and culture there's been an increasing concept of detachment. On the surface that seems a lot like the original definition of cool I said above: nonchalant and unbothered. But detachment goes further. Detachment is actually not caring about anything; not having interests. Is that cool?
I think there's a better way to be cool.
Follow your interests and be into what you're into. Don't worry about if nobody else is into it—and, on the flip side, don't worry if everybody else is into it. Be into it because you like it. Don't use it as a way to pretend you're someone you aren't—own up to the fact that you just like it. Settle that in your own mind and in how you talk to other people. Express your interest to people who share it and people who don't—not to show it off, but to be yourself. Affirm other people who do the same, whether their interests are the same as yours or not. And when people aren't cool with it, think about whether you should ignore it or whether it's the right time to say something back. Especially if they're bullying someone else.
Are you into vintage computers? Rollerblading? Christian rap metal? Building apps as a hobby? Watching videos analyzing the public infrastructure in video games? Cartoons about magical ponies? Shibuya Punk?
You know, hypothetically.
Then own it.
Or are you into crochet? Soap operas? KPop Demon Hunters? Furry art? Boy bands? Then own it.
One important note: I recognize that as a white guy in the US I have a lot of privilege. Even if people judge me for the things I'm into, the benefit of the doubt they give me outweighs it a lot of the time. Anyone in a group that has less privilege, and so is already at a disadvantage, can be put even more at a disadvantage by expressing interests that other people reject. That sucks. The most practical thing may be for you to keep an interest of yours private, or find a relatively private online space or real-life group where you can express it. If you have to, that sucks and I'm sorry you have to do that. But I would say: don't do it out of shame for what you like. Do it as an act of defiance against people who would judge you.