Historically, there was a belief that the moon's phases had an influence on human behavior, particularly on mental health. This belief was based on the observation that some people seemed to experience changes in mood and behavior during the full moon.
While this belief is now considered scientifically unfounded, the association between the moon and mental illness persisted, leading to the term "lunatic" being used to describe people with mental disorders.
However, it's important to note that using "lunatic" to refer to people with mental illness is outdated, stigmatizing, and offensive. It's crucial to use respectful and appropriate language when discussing mental health.