A colloquial phrase meaning someone is acting strangely or is not right in the head; can also mean being overly infatuated or not oneself.
彼、あんな危ない橋を渡るなんてどうかしているよ。
He's crazy to cross such a dangerous bridge.
最近ずっと残業ばかりで、どうかしているとしか思えない。
Working overtime all the time lately — I can only think something's wrong (with that situation/person).
あの子は彼のことをどうかしているみたいで、話を聞くたびに目が輝いている。
She seems infatuated with him; every time she talks about him her eyes light up.