in a world of one's own
1. In the place where you belong; in your own personal surroundings; apart from other people.
They are in a little world of their own in their house on the mountain.
2a. In deep thought or concentration.
Mary is in a world of her own when she is playing the piano.
Compare: LOSE ONESELF.
Not caring about or connected with other people in thoughts or actions. — Usually used sarcastically.
That boy is in a world all by himself. He never knows what is happening around him.
Source: A Dictionary of American Idioms