Rattleboned is a word that's not commonly used and might not be familiar to everyone. If you're looking for synonyms for this word, then a few options are available. The word could be substituted with words like emaciated, wasted, or gaunt. These words all convey the same sense of thinness or weakness and might be more commonly used than rattleboned. Other options could include the words skinny or skeletal. Each of these synonyms can be used to describe someone who is very thin or malnourished, providing context to the reader on their physical state. Regardless of which synonym you use, each will imply a sense of frailty to the person being described.