Green’s Dictionary of Slang

1. (US) anyone stupid and obstinate, often used of a horse; thus a skinhead (see cite 2001).

1916		Chicago Defender 30 Sept. 4: They sure are a bunch of hammerheads [HDAS].
1917		Van Loan ‘Egyptian Corn’ in Old Man Curry 225: [of a horse] Who wants an old hammer-head like that?
1939		L. Short Raiders of the Rimrock 85: Will Mersey was a fumble-fingered hammerhead that was bound to end up that way.
1960		Wodehouse Jeeves in the Offing 43: Something in the nature of a bohunkus or a hammerhead.
1975		G. Underwood ‘Razorback Sl.’ in AS L 1/2 60: hammerheadn One who is stubborn, obstinate.
1984		I. Doig Eng. Creek 29: You could go into a bar and still find an occasional old hammerhead who proclaimed himself nothing but a cowboy and never capable of drawing breath as anything else.
1986		C. Stroud Close Pursuit (1988) 129: Hey, hammerhead. Clean your face off [...] You’re supposed to purr, hammerhead. Not chirp.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 84/1: hammerhead n. 1 a skinhead.

2. a black person.

1970

D. Lebofsky Lex. of Phila. Metropolitan Area n.p.: Jungle bunny, chocolate bunny, burrhead, hardhead, hammerhead, rughead, nightfighter, spearchucker, hatchethead, rooster.

3. see hammerhead under hammer and tack n.