Green’s Dictionary of Slang

1. (US prison) a guard.

1925		H. Leverage ‘Dict. Und.’ in Flynn’s mag. cited in Partridge DU (1949).
1950		Goldin et al. DAUL 203/1: Spindle. (Scattered: South, mid-West, and Central U.S.) A prison guard. ‘That old spindle that was chivved (knifed) in the big crush (prison-break) slammed off (died).’.

2. (US) a manager, an owner.

2011

F. Bill ‘Cold, Hard Love’ in Crimes in Southern Indiana [ebook] ‘Between pulling doubles and hearing that old spindle’s words day after day, I cain’t take much more’.