get into someone's head

Verb

get into someone's head (third-person singular simple present gets into someone's head, present participle getting into someone's head, simple past got into someone's head, past participle (UK) got into someone's head or (US) gotten into someone's head)

1. (intransitive) To understand someone's thoughts or feelings.
   • Yet unlike many of Jackson’s supporters, he would not label his fellow senators racist. “I’m not going to try to get into the head of my colleagues to understand what’s motivating them. It’s a fool’s errand,” he told me.2022 March 24, Russell Berman, “‘I’m Not Talking About Racism. I’m Talking About Decency.’”, in The Atlantic
2. (intransitive) To have a negative impact on someone's confidence or performance; to throw someone off their game.
   • Even more incredibly, he defended it last month, brushing aside the formidable world No 1, Michael van Gerwen, in the process. The Dutchman, still overwhelmingly the man to beat, firmly denies the upstart challenger has got into his head. Which is probably an admission that he has, a little bit.2019 December 8, Jonathan Liew, “Gerwyn Price: ‘I was painted as the pantomime villain of darts’”, in The Guardian
   • Just two days after Durant suggested the Celtics got into his head, forced him to overthink and even second-guess his own capabilities, the Nets’ star appeared significantly more sure of his actions with the season on the line.2022 April 26, Kristian Winfield, “Celtics end Nets season from hell with clean sweep”, in New York Daily News