1. Suffering from lunacy; insane.
2. Of or for the insane.
3. Wildly or giddily foolish: a lunatic decision.
4. Characterized by lunacy or eccentricity.

[Middle English lunatik, from Old French lunatique, from Latin lūnāticus, from lūna, moon.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.