Green’s Dictionary of Slang

hardhead n.

[SE hard-headed; sense 2 reinforces the white cliché that one can never knock out or hurt a black man by hitting him on his head because it is too solid to damage]

1. [mid-19C–1960s] (US) a white native of rural Tennessee or Kentucky.

2. (US) alcohol, usu. whiskey.

3. [1930s+] (US black) a rebellious, non-conformist black person, a hot-tempered person.

4. [1900s] (Aus.) a regular horserace gambler.

5. [1930s+] (Aus.) a villain, a criminal.

6. [1940s+] (US) an extremely zealous person.

7. [1950s+] (US) a fool.

8. [1950s+] (US) an intransigent, uncompromising person.