in (one's) head

in (one's) head

1. Not substantial or real; having been imagined or invented. I don't know why you think I don't trust you. I can assure you that it's all in your head. This fear is just in your head—you're going to be fine.
2. Within one's brain, mind, or imagination. What possessed you to let our indoor cat go outside? I swear, I don't know what goes on in your head. After nearly four hours trying to come up with a solution, I finally had a light bulb go off in my head about what we needed to do. Ugh, this song has been stuck in my head for days—I'm so sick of it!
See also: head
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.