The antonyms for the word "good-mannered" can be numerous; some of the most common ones include rude, impolite, discourteous, ill-mannered, unmannerly, boorish, and uncivil. These are the traits that often describe those who lack etiquette or social skills, express themselves in a manner that is not acceptable in polite society, or behave in a way that is considered inappropriate. Good manners signify respect, consideration, and empathy towards others, and the absence of this quality results in the display of negative traits such as indiscipline, rudeness, and disregard for others. Therefore, possessing good manners is essential for individual, social, and professional growth, and practicing it daily is key to a successful and fulfilling life.