in one's head prepositional phrase4 •Within one's intellect, memory, or mind."I have actually gotten to revisit, in my head, all the museums I've ever gone to." — 1996 •Mentally.Captain Whalley, who seemed lost in a mental effort as of doing a sum in his head, gave a slight start. — 1902 •Within one's imagination; without basis in reality."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." — 1993 •Anxious due to overthinking an issue.
