The term "slack spined" implies weakness or lack of strength, so antonyms for this phrase can include words like strong, steadfast, resolute, and determined. Someone who is described as having a slack spine lacks motivation and energy, so antonyms can include words like dynamic, vigorous, active, and energetic. A person with a slack spine might be seen as lazy, so antonyms for this description could include words like hardworking, industrious, diligent, and ambitious. Ultimately, using antonyms for the word "slack spined" helps to emphasize positive qualities and traits in people instead of focusing on their perceived weaknesses or shortcomings.