Lunatic

A lunatic (colloquially, "loony") is a pejorative term for a person who is mentally ill, dangerous, foolish or unpredictable. The word gains its stem from the latin for moon ('luna') which denotes the traditional link made in folklore between madness and the phases of the moon. This probably refers to the symptoms of cyclic mood disorders such as Bipolar disorder or cyclothymia, the symptoms of which may also go through phases. However, there is little evidence for any causal link between phases of the moon and the progression of mood disorder symptoms.