headhunter (1958)

“Ruben Gomez, the new Phillies pitcher, a high-strung screwball thrower with a screwball attitude, has been called many other names — some unprintable. One of them is ‘head-hunter’, which was nailed on him by angry opposing players. The Puerto Rican has frequently been charged with deliberately trying to knock the batters lose [sic] from underneath their helmets.” “New Phil Hurler Tagged with ‘Head-Hunter’ Label,” Salisbury (MD) Times , December 5, 1958, p26

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1962.  “Don Drysdale, reportedly the most vicious headhunter baseball has seen since Sal Maglie’s pitching days.” (Zane Chastain, Lima [Ohio] News, May 30; Peter Morris).  Also spelled “head hunter.”