in his/her head
Je veux juste savoir ce qui se passe dans sa tête.
I just want to know what's going on in his head.
— Five Nights at Freddy's 2 (2025)
It's in his head.
Que se passe-t-il dans sa tête ?
What's going on in his head?
Un éclat de la grenade a été retrouvé dans sa tête.
A piece of shrapnel from the grenade was found in his head.
In his head.
On panique à l'intérieur, dans sa tête.
We’re panicking inside, in our heads.
— Reservoir Dogs (1992)
Sa maladie était surtout dans sa tête.
His illness was mostly in his head.
En fait, on sait pas ce qui se passe dans sa tête de beau gosse.
Actually, we don't know what's going on in that handsome guy's head.
— Crazy Rich Asians (2018)