Lunatic (noun)

1. A person who is insane.
2. A person who behaves wildly or irrationally.

Origin:

Late middle english: from latin lunaticus, from luna "moon" (because insanity was once thought to be influenced by the phases of the moon).

Examples:

1. He was locked up in an asylum for the insane, often referred to as a lunatic asylum.
2. The lunatic was screaming and ranting in the corner of the room.
3. He was accused of being a lunatic and locked away in an asylum.
4. The lunatic's behavior was too erratic for the doctors to diagnose him.
5. The lunatic's actions were dangerous and unpredictable.