It doesn't necessarly have to be an unkind thing. How about a scenario like this:
Jane, Karen, and Jim all know each other and are socializing. At the moment, Jim is standing somewhere else and only Jane and Karen are together, talking to one another.
Jane (looking over at Jim): He's so funny. I don't know what it is about Jim, but I really like him. He's a good guy.
Karen: Yeah. You know, he's a real expert on Ancient China.
Jane: Oh, I know! In fact, have you noticed that no matter what the conversation is about, he finds a way to connect it to Ancient China?
Karen: Yeah, and he gets that twinkle in his eye when he's talking about it. It's like he suddenly lights up. It's kind of cute.
Jane: Tee hee! You're right! Oh here he comes....
A bit later, they are all talking about the state of the local school district, and Jim suddenly lights up and makes a comment about how school was conducted in Ancient China, becoming energized and animated. Karen gives a little nudge to Jane, as if to say, "See? There it is! There's that cute thing he does!"
It's not negative or hateful in this case. They like Jim, quirks and all, and they enjoy seeing the way he gets when he talks about Ancient China. It's part of what makes him Jim.
If Jim sees that, and wonders what it means, then, if Karen notices that he looks confused, she can say, "Jane and I were just talking about how much you know about Ancient China, and how all kinds of other topics make you think of it. We think that's really neat that you have such a strong interest. Your whole face lights up when you talk about it. That's why I gave her a little nudge, because I just notice that you were doing it."