Thickskull is a term used to describe someone who is stubborn, unyielding, or difficult to communicate with. Although it is not a flattering term, there are many synonyms that can be used to describe this kind of behavior. Some of the most common synonyms for thickskull include obstinate, inflexible, unyielding, hardheaded, bullheaded, pigheaded, and stubborn as a mule. These words all convey a sense of resistance or unwillingness to change one's mind, and they can be used to describe people who are difficult to persuade or negotiate with. Despite their negative connotations, these synonyms can be useful when trying to describe someone's behavior or attitude.