1. Insanity, especially insanity relieved intermittently by periods of clear-mindedness. See synonyms at insanity.
2. 1. Great or wild foolishness.
   2. A wildly foolish act.
3. Archaic. Intermittent mental derangement associated with the changing phases of the moon.

[From LUNATIC.]

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.