"La pucha" is a (somewhat) elegant way to hint at a very old profession. It's a bit like saying "Qué míercoles te pasa?"... Not sure if that's clear :P
I´m not sure I get it. What do you mean a very old profession? The other expression if I understand correctly is like say "what in the heck happened?" The offensive word is changed out for a non-offensive word that sounds similar. Is that the case with "la pucha"?