If you act in an impolite, ungracious, or discourteous way, you'll be thought of as ill-mannered. It's ill-mannered to cut in front of people in line, to take a subway seat where an elderly person was getting ready to sit, or to invite everyone in your class except one person to your birthday party. Ill-mannered adds the prefix ill-, "badly," to mannered, "having manners," from manner, "way of doing things."