If a task or area of knowledge is not your department, you are not responsible for it or do not know much about it. The political issues are something else, but that's not really my department. `If you identify him, then what?' — `Not my department.' Note: You can also say that something is your department if you are responsible for it or know a lot about it. `So what do we do with him?' Admiral Polaski asked. `That, gentlemen, is our department,' Rintner said. Bill spoke expertly. This had been his department.