Green’s Dictionary of Slang

1. a black man.

1942		Z.N. Hurston ‘Story in Harlem Sl.’ in Novels and Stories (1995) 1009: Jar head: Negro man.
1983		I.L. Allen Lang. of Ethnic Conflict 50: Animal Metaphors: jar-head [the same term, a Southernism, means a mule].
1988		(con. 1930s) C.E. Lincoln The Avenue, Clayton City (1996) 171: I ain’t never seen the jarhead I’d let kick me an’ git away with it.
1989		H. Rawson Dict. of Invective (1991) 48: Jarhead has enjoyed other deprecatory meanings, especially in the American South, where it has been used to denote a black male.
2013		Chicago Trib. 17 Mar. FICT-10: ‘What the shit happened?’ ‘That fucking Jarhead Earl’s what happened [...] Robbed me of one thousand dolars’.

2. a fool, a slow, stupid person; thus jar-headed adj.

1942		in Pittsburgh Courier 18 July 7: When a jarhead lightly speils [sic] [...] just listen [...] and Wait [HDAS].
1971		Current Sl. VI.
1975		J. Carr Bad (1995) 34: Jungle, a barrel-chested, jar-headed goon with hair all over his body.
2006		J. Ridley What Fire Cannot Burn 92: Jarheads, youth wasted on Playstation and thinking cop work was nothing but a video game.