IN A WORLD BY ONESELF

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. 2b. {slang} 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./