losing oneself; being completely absorbed; forgetting oneself
Describes a state of total immersion in an activity, where one loses self-awareness. Often used with ～になる or ～で.
He threw himself into his work, completely absorbed.
子供たちは無我夢中になって遊んでいる。
The children are playing, totally lost in their game.
He became completely absorbed in the game, forgetting everything else.
I ran as if in a trance, completely forgetting myself.