Translation guide
Describes the state of being so absorbed in one's own thoughts that one is unaware of the surroundings. Japanese has several common expressions, ranging from neutral to poetic.
He was lost in thought.
To describe someone who is thinking deeply and not paying attention to what is happening around them.
A very common and natural way to say someone is lost in thought. Literally 'doing thinking things'. Can be used in many contexts.
考え事をしている implies active thinking about something specific, while ぼんやりしている often means just spacing out without focused thought. Use 考え事をしている when you want to emphasize that the person is preoccupied with thoughts.
He seems to be lost in thought (about something).
He was lost in thought and didn't notice when I called him.
A more literary and poetic expression. Often implies a somewhat melancholic or wistful state. Suitable for writing or formal speech.
She was lost in thought by the window.
Means 'to be absent-minded' or 'to be spacing out'. It can imply being lost in thought, but often with a nuance of not thinking about anything in particular. More casual.
I was lost in thought during class and got scolded by the teacher.
Literally 'sinking in thought'. A very literary and emotional expression, often used for deep, heavy contemplation or sorrow.
He was sunk deep in thought and didn't say a word.
彼はぼんやりしている。
He is spacing out.