We use this expression to say that someone is obsessed with something or always thinks about something. It’s often negative.
You can use this phrase in ………… language.
A: I get so bored talking to Jaromír. All he’s interested in is frisbee.
B: I know. He’s got frisbee on the brain.
A: Are you coming to the party on Friday? Jana will be there. Jana’s great.
B: Will you shut up about Jana! You’ve got her on the brain!