to be physically present in one location while one's thoughts or heart are longing to be somewhere else

他在课堂上听讲，可由于惦记着下午的比赛，身在曹营心在汉。

to be physically in one camp while remaining loyal to another, originally referring to Guan Yu staying with Cao Cao while remaining loyal to Liu Bei

他虽然在对手的公司工作，但身在曹营心在汉，始终想回到原单位。