Green’s Dictionary of Slang

duke n.4

1. [1920s] a champion, one of the best.

2. [1930s+] a tough, dominant individual, a leader or boss, esp. in the criminal world.

3. [1940s] (US prison) the warden.

4. [1970s+] (US gay/prison) a predatory prison homosexual.

5. (US black) used as a term of address.