Ill-mannered means behaving rudely or without consideration for others, showing a lack of good manners. An ill-mannered person might interrupt constantly, chew with their mouth open, or shove past people in line without saying “excuse me.”

The word describes someone who either never learned proper behavior or chooses to ignore it. An ill-mannered dinner guest might reach across the table for food instead of asking for it to be passed, or talk loudly while others are trying to speak. An ill-mannered classmate might make rude comments about someone's presentation or refuse to say “thank you” when someone helps.

Notice that ill-mannered is stronger than just saying someone made a mistake or forgot their manners once. It suggests a pattern of rude behavior. Everyone has off days or forgets to say “please” sometimes, but an ill-mannered person regularly acts without courtesy or respect for others.

The opposite would be well-mannered: someone who consistently treats others with respect and follows basic rules of courtesy. Good manners help people get along and show consideration for others, which is why ill-mannered behavior stands out so noticeably and can make others uncomfortable.