in a world of one's own

[in a world of one's own] or [in a world by oneself] 1. In theplace where you belong; in your own personal surroundings; apart fromother people.

They are in a little world of their own in theirhouse on the mountain.

2a. In deep thought or concentration.

Maryis in a world of her own when she is playing the piano.

Compare: LOSEONESELF. 2b. {slang} Not caring about or connected with other peoplein thoughts or actions. - Usually used sarcastically.

That boy isin a world all by himself. He never knows what is happening aroundhim.