Definition of go crazy
1 : to become mentally ill : to go insane She went crazy and started to think everyone was trying to kill her. —usually used in an exaggerated way I must be going crazy. I can't find my car keys anywhere.
2 : to act in a way that is out of control : to act wildly We were just talking when he suddenly went crazy and started screaming and breaking things. The crowd went crazy when the team won the championship.