Green’s Dictionary of Slang

(US prison) solitary confinement; forfeiture of privileges; also as v.

1933		Ersine Und. and Prison Sl.
a.1949		N. Algren Little Lester’ in Entrapment (2009) 94: Frankie [...] worked down the tier slowly, taking the jibes of the deadlocked without reply.
1949		N. Algren Man with the Golden Arm 208: Deadlock [...] meant no yard privileges, no cigarettes, no newspapers and no mail; no candy, no card playing.
1950		Goldin et al. DAUL 57/1: Deadlock, v. (P) To lock In a cell for punishment.
1955		‘Blackie’ Audett Rap Sheet 173: So, I was kept in deadlock – in my cell, and alone.
1967		S. Harris Hellhole 47: She had me deadlocked – put into solitary confinement.
1974		E.H. Hunt Undercover 305: I was in the jail’s disciplinary section under deadlock, something reserved normally for only the most hardened and refractory prisoners.
1980		G. Liddy Will 290: The shower area came first, then the individual cages. These were the ‘deadlock’ cells; one stayed in them twenty-four hours a day.
1992		Bentley & Corbett Prison Sl. 10: Deadlock When an inmate is on deadlock, he is locked in his cell for a certain period of time.