Green’s Dictionary of Slang

buck adv.

[buck adj.1 ]

(US black) utterly, totally, completely.

[US]Hip-Hop Connection Jan. 75: Charging round the woods, getting buck-ass naked and bonding in the way that only grown men know best.
[US](con. 1962) J. Ellroy Enchanters 20: Carole Landis, buck nude on a gurney.