on the/(one's) brain(redirected from on their brain)
on the/(one's) brain
Occupying one's sole or most prominent attention or interest; forming an obsession or preoccupation for one. Tom's just got football on the brain—he's not really interested in anything else. The history of this town has been on my brain ever since high school, so I'm delighted to finally have a creative outlet to explore those ideas.
on the brain
Obsessively in mind: The coach has winning on the brain.