In (one's) own (little) world American British prepositional phrase
It can also be phrased as "in a world of one's own".
To indicate one is lost in deep thought or concentration and does not aware of what is happening around him/her
The teacher got angry when Jane looked outside the window and was constantly in his own world during the French lesson.
Every time our kids play the game, they are always in their own little world.
Jenny is a poetic girl, she always lives in her own world when standing in front of the grand mountains.
Be very nearly or inevitably lost, departed or deceased
1. To focus on one task until carry it out.
2. To work hard to pay educational fee.
Lost among a confusing mix of things
To disappear.
To be influenced by someone because you are strongly attracted to them.