What does "a nail on the head" mean? And when do you use it?
This is just speculation, but I believe this phrase may have been made-up for the show in order for Ariana's song to sound silly. It also could be a reference to the common phrase "hit the nail on the head." This phrase means to do or say something perfectly; exactly correct. You can use "hit the nail on the head" when someone said or did something perfectly. However, this is just speculation and could mean something completely different.