1. insanity; mental disorder.
2. intermittent insanity, formerly believed to be related to phases of the moon.
3. extreme foolishness or an instance of it.
4. Law: unsoundness of mind sufficient to incapacitate one for civil transactions.
Etymology: lunatic (from Middle English lunatik < Old French lunatique < Late Latin lūnāticus, “moonstruck”, from Latin lūna, “the moon”) + -acy (a suffix of nouns of quality, state, office, etc., from Latin -ācia, -ātia).
[Lenka Simeckova - Janitor of Lunacy]