From an interview with David Graeber:
Well it’s fairly straightforward: shit jobs are just bad jobs. Ones you’d never want to have. Back-breaking, underpaid, unappreciated, people who are treated without dignity and respect… The thing is for the most part, shit jobs aren’t bullshit, in the sense of pointless, nonsensical, because actually they usually involve doing something that genuinely needs to be done: driving people around, building things, taking care of people, cleaning up after them…
Bullshit jobs are most often paid quite well, involve nice benefit packages, you’re treated like you’re important and actually are doing something that needs to be done — but in fact, you know you’re not. So in that way they’re typically opposites.