The antonyms for the word "most good-mannered" would be "ill-mannered," "uncivil," "impolite," "rude," "crude," "discourteous," "uncouth," "vulgar," "impudent," "insolent," "brusque," and "abrupt." These words describe individuals who lack basic social etiquette and cultural sensitivity. Often, their actions or behaviors could offend others or violate social norms. Consequently, being "most good-mannered" is significant for building harmonious relationships with others, whether at work or in personal settings. Therefore, to avoid becoming an outcast or undesirable company, one should cultivate good manners and cultivate a positive attitude towards others, regardless of their background or beliefs.