If your dad used to play in a punk band, but is now a computer programmer, you could call him an erstwhile punk rocker. Erstwhile means "former."
Erstwhile comes from Old English and is a formal way to say "former." You can go to your class reunion 20 years after you graduate high school and see your erstwhile friends. That means that they had been your friends, but you lost touch with them, so that they weren't friends of yours anymore. Can you think of some people that you wish were erstwhile classmates of yours?