Be (not) (one's) department spoken informal
If you say that something is (not) your department, you mean that it's (not) your area of expertise or responsibility.
I'm afraid that question is not my department. Sorry!
My particular area of expertise is database design. Don't ask me about alternative energy. That's Mike's department.
Are you cracking the password of this system? This is definitely your department.
Don’t ask me about why! It's not my department.
Be outside the ability or willingness of someone to care more about a particular person or issue
Being looked after or protected by someone
It is used to tell someone that it is time for them to take action or make a decision.
Know a lot about something
The verb "be" should be conjugated according to its tense.
Daughter: So I really want all my friends to have a great time. I was thinking it would be a good idea to have some candy out on the tables for people to snack on.
Mother: Well, if you want them to really enjoy themselves, candy is dandy but liquor is quicker.