Lunacy (noun) Definition, Meaning & Examples

noun, plural lu·na·cies.

insanity; mental disorder.
intermittent insanity, formerly believed to be related to phases of the moon.
extreme foolishness or an instance of it: Her decision to resign was sheer lunacy.
unsoundness of mind sufficient to incapacitate one for civil transactions: a former legal term.

noun plural -cies