Rattlehead is a slang term that is used to describe a person who is unpredictable or behaves in a bizarre manner. There are several synonyms for this word, including loony, nutcase, nutjob, and crazypants. These words all suggest a certain level of instability or mental illness. Other synonyms for rattlehead include kooky, wacky, and zany, which suggest a more lighthearted or humorous approach to strange behavior. Ultimately, the synonym you choose to use will depend on the context and tone of your writing. Whether you opt for a serious or humorous term, it's important to consider how your words will be received by your audience.