정신(精神) [n] mind, the power of feeling, thinking about, or judging something.

차리다 [v] come to one’s senses, To recover one's energy, consciousness, etc.

informal/casual – 정신차려! 

>> This expression is used to remind others to think or act properly when they are in a state that seems unusual or abnormal.

>>Here, 정신 is talking about the ability to feel, think and judge things. So, this expression is used when talking to someone who is thinking, saying, or doing something they would not do under normal circumstances, reminding them to be able to discern between right and wrong.