Green’s Dictionary of Slang

(US prison) homemade alcohol.

1933		C.W. Willemse Cop Remembers 139: Did you ever drink watermelon buck? Well, take mah tip, Son, and don’t. That’s the stuff that makes a rabbit look for a dawg.
1938		C.H. Matschat Suwannee River 135: Cane beer, with a sweet-sour taste, would be made from the fermented skimmings and, later, would form excellent ‘buck’ for a wildcat still [DA].
1951		J. Blake letter 23 Dec. in Joint (1972) 27: I have started a little bootlegging operation, making ‘buck’ (prison liquor) out of cornbread, cane syrup and water.
1998		M. Baker Bad Guys 22: Selling marijuana, smoking marijuana, drinking buck, selling buck.
2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 Buck: Homemade alcohol (Florida) [...] Buck Master: One who makes homemade alcohol. See ‘pruno.’.