Green’s Dictionary of Slang

1. (US Und.) a prison.

1914

Wash. Post 15 Mar. Misc. 1/5: A ‘peetman’ is a safeblower, and when he blows a safe he ‘cracks a joint.’ If he is caught and taken to the station in the ‘paddy wagon’ he may be given ‘six months in the buck’ or a ‘long bit’ or a ‘short bit.’.

2. (US prison) a sentence.

1944

C. Himes ‘A Penny for Your Thoughts’ in Coll. Stories 57: Dannie Lambright was the name of her husband serving a double murder buck in Houston.