1. a person who is mentally ill
Usage: archaic; offensive
2. a person who behaves in a very foolish or reckless way
Usage: informal

1. extremely foolish or unreasonable
Usage: informal

Examples

• The driver was speeding like a lunatic on the highway.
• That’s a lunatic idea that will never work.
• He called his brother a lunatic for jumping off the cliff.
• The lunatic scheme cost the company millions.
• She thought he was acting like a complete lunatic.
• The lunatic driver nearly caused an accident.