Green’s Dictionary of Slang

duker n.2

[duke n.3 (1)/duke v.1 (3)]

1. (US carnival) a fortune teller.

[US]J.E. Dadswell Hey, Sucker 101: duker ... fortune-telling palmist.

2. (US) a boxer, a fist-fighter.

[US]J. Horton ‘Time and cool people’ in Trans-action 4 n.p.: Survival requires that a hustling dude know who is cool and uncool (who can be trusted); who is in power [...]; who is the ‘duker’ or the fighter (someone to be avoided or someone who can provide protection).
[US]E. Torres Carlito’s Way 137: I coulda been a pretty good duker.