Green’s Dictionary of Slang

1. a champion, one of the best.

1920		R. Lardner Big Town 147: He’s the duke of them all when he lays off the liquor.
1921		R. Lardner ‘A Frame-Up’ in Coll. Short Stories (1941) 419: He certainly is Duke of the Cuckoos and the world’s greatest sap.

2. a tough, dominant individual, a leader or boss, esp. in the criminal world.

1939		in A. Banks First-Person America (1980) 179: Even when some duke tells you about some job in a big office, you don’t try for it.
1947		N. Algren Neon Wilderness (1986) 129: Some duke was just in askin’ for you, Venus.
1967		F. Salas Tattoo the Wicked Cross (1981) 313: He’s a duke, Aaron [...] Nobody expects you to fight him fair.
1976		M. Braly False Starts 45: It was a rare fish able to carry this off with any style, a big duke from L.A. or Frisco.
1981		E. Bunker Little Boy Blue (1995) 128: He was one of the ‘dukes’ – one of the best fighters.
1990		A. Vachss Blossom 108: The duke. The head man.
2000		T. Blacker Kill Your Darlings 192: These were dealers, drug dukes enacting their own little bloody border disputes.

3. (US prison) the warden.

1949

Monteleone Criminal Sl. (rev. edn).

4. (US gay/prison) a predatory prison homosexual.

1972		B. Rodgers Queens’ Vernacular 155: The sexually dominant prisoner [...] is called duke (fr carnival sl duke it out = to hand something over).
1977		P. Hamill Flesh and Blood (1978) 32: You are what is known as a white fuck. A white fuck don’t do easy time. You can be a duke or a dip, a fuck boy or a jailhouse punk.

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

2012

‘Dutch’ ? (Pronounced Que) [ebook] Shameeq brushed past him, which made Black look up. ‘Pardon me, duke,’ Shameeq said.