From Longman Dictionary of Contemporary Englishhave something on the brainhave something on the braininformalTHINK ABOUT to be always thinking about something I’ve got that song on the brain today. → brain
Examples from the Corpus
have something on the brain• You always have food on the brain.• But the festering problem may have effects on the brain, just as it can elsewhere in the body.• Must have maggots on the brain.• It's unbelievable - you have sex on the brain 24 hours a day! 