expression, godan verb (-ru)
slap in the face
A set phrase meaning to slap someone's cheek. The verb 張る here conveys a sharp, striking action.
彼は怒って相手の頬を張った。
He got angry and slapped the other person in the face.
そんなことを言うなら、頬を張るぞ。
If you say that, I'll slap you in the face.