get into (one's) head
1. To begin to understand how one thinks. You need to get into the criminal's way in order to find a way to stop him.
2. To intentionally disrupt or undermine one's focus, confidence, or performance. The opposing coach called a time out at the last second to try to get into the kicker's head.
get into one's head
Also, get or take it into one's head . Form an impression, idea, or plan. For example, What strange idea has she got into her head? or He took it into his head that you want to quit. [Late 1600s] Also see get through one's head.