An ill-mannered person behaves badly in social situations: talking over others, grabbing food, skipping thank-yous, or treating staff as if they were invisible. It is about manners — the small rules of being together — not about being evil. Older speakers and more formal writing still use it; younger people more often say “rude” or “has no manners.” Though actually, what counts as ill-mannered changes with place and age. Interrupting might feel lively in one family and shocking in another. In a Taiwan classroom this word usually maps onto the 沒禮貌 you already feel in your bones: not a crime, but a social stumble people notice.