This is a very difficult question ;) you can use this word in various situations but in very negative and impolite ways. I think you can use it when you look down on someone or something. 기대는 개뿔 - nothing to expect / better not to expect ex) What he said was all rubbish (개뿔같은 소리) / He doesn't know shit (개뿔도 모르다. I hope this may help you!

It is never expected thing, rude and violent word. ex) 개뿔도 모르다. / 개뿔도 없다.