Crappy is a rough, everyday way to say that something is very bad — a film, a phone, the weather, a job, even how you feel. It is not a careful word. People say it when they are annoyed or when they do not care about sounding polite. It sits on the rude side of informal: fine with friends, not fine in a school essay, a job interview, or when you are speaking to someone you should show respect to. The bite comes from crap, which people already use for rubbish and, more crudely, for waste from the body. Add -y and you get “like that / full of that.” In real talk the strength slides around. Sometimes crappy means truly awful. Sometimes it only means “not great,” the way someone might call a cheap meal crappy without meaning it is disgusting. American English uses it a lot; other kinds of English use it too, just a bit less in careful speech. Older people, and people who dislike swearing, may find it uglier than younger speakers do. It describes things and situations more often than people. Calling a person crappy happens, but it sounds like a slap.