The phrase "most fast-and-loose" implies a disregard for rules or morals, making it difficult to find exact opposites. Nevertheless, some antonyms for this phrase could include words like careful, cautious, responsible, and attentive. These words suggest a focus on rules and caution, unlike the reckless nature of being "most fast-and-loose." Other antonyms could be ethical, principled, or honest, indicating a strong sense of morality and integrity rather than a willingness to take risks without regard for others. In short, antonyms for "most fast-and-loose" can be found among words that connote carefulness, responsibility, and uprightness.