"More rattleboned" implies a state of extreme frailty, weakness, and skeletal thinness. On the contrary, antonyms for this phrase could refer to a person or thing that is robust, strong, and sturdy. Some words that come to mind are fortified, substantial, robust, healthy, muscular, and well-built. These terms suggest that the subject is not only physically hearty but can also endure challenges and problems with grace and ease. While "more rattleboned" might describe an individual who is malnourished or ill, its antonyms emphasize a state of wellness and vitality, indicating that the person or entity is flourishing and prospering.