A lunatic's a commonly used term for a person who's mentally ill, dangerous, foolish or unpredictable: a condition once called lunacy. Linguistic roots The word lunatic's borrowed from italian "lunacus", Italian which gains its stem from "luna" for moon, 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 pha… (More on Lunatic)