Green’s Dictionary of Slang

duke n.1

[abbr. rum duke under rum adj.; 20C+ use is US although its root may lie in the more recent dude n.1 (2)]

1. (UK Und.) a lookout.

[UK]Greene Art of Conny-Catching n.p.: duke He that standeth to watch In high lawe.

2. a showy, ostentatious man.

[UK]B.E. Dict. Canting Crew.
[UK]Farquhar Recruiting Officer II iii: You are a justice of the peace, and you are a king, and I am a duke, and a rum duke, a’n’t I?
[UK]Swift Polite Conversation 80: He’s a queer old Duke, by my conscience.
[UK]Grose Classical Dict. of the Vulgar Tongue n.p.: Duke, or rum duke, a queer unaccountable fellow.
[UK]Lex. Balatronicum [as cit. 1785].
[UK]Kipling ‘The Flag of Their Country’ in Complete Stalky & Co. (1987) 217: Here’s the giddy Dook. Golly, what a dewlap!
[US]Noble Sissle ‘Mirandy’ [lyrics] The dark-town duke of Jacksonville was a Jackson nicknamed Sandy, / This dusky duke from Jacksonville had a gal they called Mirandy.
[US]Howsley Argot: Dict. of Und. Sl. 17: duke a high class swell dressed international (usually) crook, who has entree and moves in high society.
[US]N. Algren Neon Wilderness (1986) 247: Sometimes a man looked like a regular duke and didn’t have a dime.
[US](con. 1937) C. Chessman Cell 2455 91: There were those who wanted his job, who wanted to make a reputation as tough guys, bad dukes.
[US](con. 1970) J.M. Del Vecchio 13th Valley (1983) 301: Yo bes not be able to see this duke [...] Ef yo see em, then so ken a good.