in one's head

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in one's head

1. Used other than figuratively or idiomatically: see in,‎ one's,‎ head.
2. (idiomatic, of information, a plan, etc.) Within one's intellect, memory, or mind.
   • 1996 March 24, Frank Bruni, “The Grunge American Novel”, in The New York Times‎^[1], archived from the original on 26 June 2019, retrieved 9 December 2016:

     "I have actually gotten to revisit, in my head, all the museums I've ever gone to."

   • 2013 August 10, Michael Calvin, “Sport: Scouting system that reveals David Moyes' mind”, in Independent‎^[2], UK, archived from the original on 28 July 2016, retrieved 9 December 2016:

     The old-school way, with the chief scout having it all in his head, gave no continuity. If he gets run over by a bus he takes all the knowledge with him.

3. (idiomatic, of a false belief, delusion, etc.) Within one's imagination; without basis in reality.
   • 1993 November 28, John Schwartz, “Caution: Children at play on information highway”, in The Washington Post‎^[3], archived from the original on 30 September 2025:

     "Kids disappear into imaginary relationships all the time" . . . but "usually there's a limit to how far that imaginary relationship can go, because it's all in their head."

   • 2006 October 22, Pam Belluck, “Living With Love, Chaos and Haley”, in The New York Times‎^[4], archived from the original on 10 March 2016, retrieved 9 December 2016:

     When a neurologist ruled out medical causes like Lyme disease, Ms. Abaspour recalled, her husband said, “I think we should just give her a placebo — it’s all in her head.”

   • 2014 April 5, Gwilym Mumford, “True Detective recap: season one, episode seven”, in The Guardian‎^[5], UK, archived from the original on 5 March 2016, retrieved 9 December 2016:

     The tape was the piece of evidence that assured Cohle that this grand conspiracy wasn't all in his head.

4. (idiomatic, mathematics, of performing a calculation) Mentally.
   • 1902, Joseph Conrad, chapter 5, in The End of the Tether:

     Captain Whalley, who seemed lost in a mental effort as of doing a sum in his head, gave a slight start.

5. (idiomatic) Anxious due to overthinking an issue.

• In the senses of "within one's mind" and "within one's imagination", sometimes preceded by all as an intensifier emphasizing that the indicated information, belief, etc. is entirely and exclusively within one's mind or imagination.