It seems to be pretty much a poetic commonplace, and perhaps symbolic more of "throwing over the traces" than of prostitution in songs of this sort. Dysters, of course, are simply Dyers. "Leaboy" (there is only one; the -s indicates the possessive form), Light Bob, Lichtbob and so on occur; and sometimes a ploughboy. Explanations differ: a "lichtbob" is sometimes taken to be a soldier, and a "leaboy" a herdsman. I don't know if there is a definitive answer.