Here's an explanation which was translated by Google translator:

One interpretation assumes an origin from Rotwelsch - more precisely - from the language of the Sinti and Roma (Romani). In Romani, "bokh" means hunger. However, a connection to youth language towards the end of the 1970s is hardly conceivable. Presumably, it is a matter of coincidence. Since the buck is considered to be instinctive and the "buck" (pejorative for "lustful man") has been known for some time, it may have been an autonomous development in the sense of "lust" (double sense of the word!) = "to have a buck".