Rattlebrains can be defined as an individual who is absent-minded, foolish or scatterbrained. However, there are several synonyms for the term. These include words like airhead, scatterbrain, dimwit, space cadet, and knucklehead. Other synonyms for rattlebrains include daft, witless, addle-pated, harebrained and muddleheaded. Each of these words describes someone who is not very smart, forgetful or lacks common sense. It is important to be aware of these synonyms as they can come in handy when writing or speaking about someone who lacks intelligence or common sense. Overall, while these words may seem negative, they can be helpful in describing someone's behavior or personality accurately.