Green’s Dictionary of Slang

captain; often as a term of address.

1829		Yankee Apr. 120/3: Cap’n Jessamine had got kicked out o’ bed by his wife [OED].
1850		‘Ned Buntline’ G’hals of N.Y. 40: You’ll tell the cap’n [...] that when he sees me again, he’ll have a fellow to perpose.
1866		‘R.M. Devens Pictorial Bk Anecdotes 408/2: I pulled my hat over my eyes and jogged along on the Cap’ns horse [...] with this basket on my arm.
1908		R.S. Baker Following the Colour Line 63: The round term ‘Boss,’ or sometimes ‘Cap,’ or ‘Cap’n’. To this the white man responds with the first name of the Negro.
1929		G. Johnson John Henry in Botkin (1944) 142–6: But dat ain’t all, cap’n.
1962		P. Crump Burn, Killer, Burn! 43: Shune, suh? Shine ’em up Cap’n?