godan verb (-ru), intransitive verb
be lost in thought; be immersed in reverie
Describes being deeply absorbed in one's own thoughts, often with a nuance of brooding or daydreaming.
彼は窓の外を眺めながら、思い耽っていた。
He was lost in thought while gazing out the window.
一人で部屋にこもっ
Time spent alone in a room, immersed in reverie, is also important.