Definition
This idiom indicates that someone is thinking about or focusing their attention on something specific.
心系於…、心在…上
She has been distracted all day; her mind is clearly on something.
她整天心不在焉，顯然她的心思在某件事上。
He always gets distracted during exams; his mind is always on something else.
他考試時總是分心，他的心思總是在別的事情上。
I tried to talk to him, but his mind was on something else, and he wasn't listening.
我試著和他說話，但他的心思在別處，他沒有在聽。