The term "most slack-spined" refers to someone who is weak-willed or lacking in determination. Antonyms for this phrase would include strong-willed, determined, resolute, and steadfast. Other antonyms could include reliable, dependable, trustworthy, and committed. A person who is the opposite of being slack-spined is someone who is proactive, assertive, and takes charge of their life. They have a strong sense of purpose and are driven to achieve their goals. The antonyms for "most slack-spined" reflect qualities that are essential for success in life, such as persistence, perseverance, self-discipline, and confidence.