Green’s Dictionary of Slang

(US black/prison) someone in need of mental health treatment; an eccentric, a crazy person.

2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 5150: A person needing mental health treatment, based upon the California Welfare and Institutions Code section for civil commitment. Other states will use different numbers, such as ‘730’ in New York.
2015		P. Beatty Sellout (2016) 78: I should’ve hospitalized him right then and there. Called the police and had him 5150’d.