When something is hellish, it is extremely bad or extremely hard to live through — a hellish day at work, hellish traffic, a hellish exam week. People are not carefully talking about Hell the religious place; they borrow that dark picture to say “this is as bad as it gets.” It is informal, a little strong, and more common in speech than in school writing. It is not quite a swear word by itself, though actually it still carries a spark of one, so some families and classrooms prefer awful or terrible. You can still use it closer to the old meaning, “like Hell,” in stories or jokes, but everyday talk almost always means “brutal, miserable, too much.”