Green’s Dictionary of Slang

a-buck adv.

(US gay) lying on one’s back with one’s legs over one’s head to enable both anal intercourse and kissing.

[US]R.O. Scott Gay Sl. Dict. [Internet] a-buck: a position for anal intercourse: laying with legs thrown over the head in the air exposing the ass-end to the active partner. [This position lets one kiss his partner.].