A state of inattention or absent-mindedness; being mentally elsewhere and not focused on the present situation.
会議中、彼は上の空でほとんど話を聞いていなかった。
During the meeting he was absent-minded and hardly listened.
試験中に上の空になると、簡単な問題でもミスをする。
If you become absent-minded during an exam, you can make mistakes even on easy questions.
母は子供のことを考えていて、上の空だった。
My mother was thinking about the children and was absent-minded.
彼女は窓の外を見て上の空になった。
She looked out the window and went into an absent-minded state.