Green’s Dictionary of Slang

buck adv.

(US black) utterly, totally, completely.

2000		Hip-Hop Connection Jan. 75: Charging round the woods, getting buck-ass naked and bonding in the way that only grown men know best.
2023		(con. 1962) J. Ellroy Enchanters 20: Carole Landis, buck nude on a gurney.

In phrases

buck-ass(ed) naked (adj.)

(US) absolutely naked.

1956		‘J. Little’ Somewhere between the Two 139: ‘If everyone walked around buck-ass naked, there wouldn’t be any fun’ [Simes:DLSS].
1969		DARE.
1974		‘C. McCandless’ Rock Hard 109: We were all standing there, buck-ass naked, our joints up, standing out, high in the air [Simes:DLSS].
1990		Handjobs (CA) Oct. 10: He was leaning against a tree, buck ass naked except for his boots [Simes:DLSS].