na-adjective, noun
inattention; absent-mindedness; spacing out
Describes a state where one's mind is elsewhere, not paying attention to the immediate surroundings or conversation. Often used with する or だ.
授業中、彼は上の空で窓の外を見ていた。
During class, he was spacing out and looking out the window.
話
Even when I spoke to him, he was absent-minded and didn't reply.