Also, on the brain. In one's thoughts, preoccupying one. For example, The book prize has been on my mind, but I haven't been able to discuss it with you. It is often put as have something on one's mind (or the brain) meaning “be preoccupied with something,” as in I didn't mean to be rude; I just have a lot on my mind right now, or John has nothing but girls on the brain. [Mid-1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.