The idiom "mad as a hatter" is often used to describe someone who is irrationally angry or insane. However, there are several other phrases that can be used as synonyms for this expression, such as "crazy as a loon", "nuts as a fruitcake", "batty as a belfry", and "off their rocker". These idioms are typically used to describe someone who is exhibiting bizarre or abnormal behavior and may be suffering from a mental illness or emotional instability. Regardless of which phrase is used, it is important to treat individuals with respect and empathy, even if their behavior seems strange or unpredictable.