Green’s Dictionary of Slang

1. time off for good behaviour.

1872		Illinois Laws 1871–2 295: For more than four offenses, the warden shall have power to deprive him [a convict], at his discretion, of any portion or all of the good time that the convict may have earned [DA].
1900		Flynt & Walton Powers That Prey 129: He went to the Stir originally with the idea of getting all the ‘good time’ that the law allows a man who has been sentenced to four years.
1912		A. Berkman Prison Memoirs of an Anarchist (1926) 478: The law allows five months ‘good time’ on every year beginning with the ninth year of a sentence.
1931		G. Milburn ‘Convicts’ Jargon’ in AS VI:6 438: good time, n. Time deducted from a sentence. ‘I get seven days a month off good time, which will leave me out next Summer.’.
1949		Monteleone Criminal Sl. (rev. edn).
1951		D. Dressler Parole Chief 117: His sentence, less ‘good time,’ was served.
1963		in T.I. Rubin Sweet Daddy 2: Nine months. But good time – I’ll do it in six.
1977		B. Jackson Killing Time 223: I finagled around and spent four or five hundred dollars buying good time and I got out about six or eight months early.
1986		J. Ellroy Silent Terror 65: Counting the ‘good time’ and ‘work time’ you will receive for working as a trusty, you have 9 1/2 months in jail to endure.
1989		J. Morton Lowspeak.
1994		N. McCall Makes Me Wanna Holler (1995) 151: Inmates who went through the program could earn ‘good-time’.
2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 Good Time: Credits earned toward one’s sentence. In California, good time (one day for two served) credits are awarded for prisoners in certain situations, such as those who are willing to work but unassigned.

2. a jail sentence that is suffered without any particular problems.

1975		E. Torres Carlito’s Way 115: I do good time, don’t go around cryin’ and wailin’.
1990		R. Campbell Sweet La-La Land (1999) 58: Going to miss you, Younger. You done good time.