in a world of (one's) own

Consumed by one's thoughts, imagination, etc., to such a degree that one is unaware of or insensitive to one's surroundings or the outside world in general. It's no use trying to talk to Helen about something like this—she's always in a world of her own. Sorry, what did you say? I was in a world of my own there. A: "Hey, did you hear the phone ring?" B: "Oh wow, no, I didn't. I guess I'm in a world of my own or something."

*in a world of one's own

Fig. aloof; detached; self-centered. (*Typically: be ~; get [into] ~; live ~.) John lives in a world of his own. He has very few friends. Mary walks around in a world of her own, but she's very intelligent.