Green’s Dictionary of Slang

(US prison) a prisoner convicted of a crime categorized as ‘anti-social’, e.g. hooliganism, rather than violent or property crimes.

1935		A.J. Pollock Und. Speaks 1/1: Accidental, an inmate convicted for a social crime (prison).
1960		(con. 1950-1960) R.A. Freeman Dict. Inmate Sl. (Walla Walla, WA) 2: Accidental – one who has been convicted of a sex crime, and is usually shunned by other inmates.