A "John" (man who has sex with prostitutes) who regularly has sex with one prostitute in particular. The reason he's called a "bounce buddy" is because the prostitute thinks of him as a buddy who has sex with her (less likely him) and then "bounces" (goes away without saying much)
Crystal the prostitute rostitute: I have a guy/gal who at least once a month pays to have sex with me. Each time after he's had sex with me, he leaves without saying much. He's a bounce buddy.
by Before Watchmen: Nite Owl 1 August 3, 2019