Green’s Dictionary of Slang

1. (US) a person who tracks down wanted criminals.

1927		‘Max Brand’ Pleasant Jim 287: ‘You damn head-hunter!’ said he.
2014		D.B. Flowers Bangs 50: McDonald was a gung-ho working cop, a genuine ‘headhunter,’ as Bangs and others referred to them, meaning a cop who loved to bust people.

2. (US) a police officer who reports on another officer.

1951		C. Stoker Thicker ’n Thieves 105: One of the head hunters (personnel investigators probing into a police officer’s suspected misconduct) [...] says that personnel has enough evidence on a certain vice squad sergeant to send him to San Quentin penitentiary.
1967		R. Conot Rivers of Blood 235: In order to insure good conduct, he operated an investigative section of 19 ‘head hunters,’ who circulate anonymously, monitoring the behavior of deputies.
1975		J. Wambaugh Choirboys (1976) 138: I wonder if Lieutenant Grimsley and all them IAD headhunters get a finder’s fee when they nail a cop.
1981		J. Wambaugh Glitter Dome (1982) 81: Neither Captain Woofer nor the Internal Affairs headhunters had been able to put it together.
1992		P. Earley Hot House 320: ‘These headhunters will ruin your career, just like they did Geouge because he slapped a Cuban’.
2014		D.B. Flowers Bangs 19: Cops like Flowers weren’t just shit-bum headhunters out to make a collar or cause people trouble simply because they could.

3. (Aus. Und.) a criminal who specializes in preying on other, successful and thus wealthy, criminals.

1991		M.B. ‘Chopper’ Read Chopper From The Inside 77: I am not a ‘bounty hunter’ as I have been called. The criminal term for someone like me is a ‘headhunter’. The term ‘headhunter’ is a purely Australian criminal slang term for someone who lives off the big crooks.
2004		M.B. ‘Chopper’ Read Chopper 3 4: I am a head hunter, not a perverted killer.