Green’s Dictionary of Slang

headbeater n.

also headbreaker, headbuster

1. (US) a thug, incl. one who works for a ruthless criminal.

Indianapolis News 20 Sept. 1/3: A vicious gang of teenage head-beaters.
Hal Ellson ‘I Didn’t See a Thing’ in Tell Them Nothing (1956) 131: He may be grinning but he’s a head-buster.
[US]T. Philbin Under Cover 38: What I want to do is to get his chief headbreaker – that’s a black guy named Ace.

2. (US black) a brutal police officer or prison warder.

[US]Lincoln Jrnl Star (NE) 29 Mar. 2/5: Other demands of the prisoners [...] Dismissal of all guards that ‘we can prove to be sadists of head-beaters’.
[US]Miami Dly News-Record (FL) 10 Aug. 4/7: ‘Don’t know which block that number is on, man, you go ask the headbeater’ [...] In his [i.e. a teenager] lingo ‘headbeater’ means policeman.
[US]E. De Roo Big Rumble 30: You don’t like bulls and headbeaters anymore than we do. [Ibid.] 95: He ain’t no screw. He ain’t no headbuster.
[US] in S. Harris Hellhole 137: She know about how the lousy headbreakers kick me in the belly.
[US]J.D. Horan Blue Messiah 64: The food’s lousy and the guards are queers and head busters.
[US]T.M. Kochman ‘The Kinetic Element in Black Idiom’ in Kochman Rappin’ and Stylin’ Out 163: Policemen are ‘headbreakers’ and the white power structure is the ‘whips,’ the latter an acronym derived from white power structure.
[US]E. Torres Q&A 50: I’ve been a headbuster, I don’t deny it.
Blakenship Blood Stripe 232: I’ll come back with some of the best head-busters in the MP’s .