lunacy

American Heritage® Dictionary of the English Language, Fourth Edition

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

Century Dictionary and Cyclopedia

1. n. The kind of intermittent insanity formerly supposed to be subject to the change's of the moon; hence, madness in general; any unsoundness of mind. See insanity.
2. n. In law, unsoundness of mind sufficient to incapacitate for civil transactions. The usual test is incapacity to manage one's own property and affairs.
3. n. Synonyms Derangement, Craziness, etc. See insanity.

Wiktionary

1. n. The state of being mad, insanity
2. n. a cyclical mental disease, apparently linked to the lunar phases
3. n. insanity implying legal irresponsability.
4. n. Something deeply misguided.

GNU Webster's 1913

1. n. Insanity or madness; properly, the kind of insanity which is broken by intervals of reason, -- formerly supposed to be influenced by the changes of the moon; any form of unsoundness of mind, except idiocy; mental derangement or alienation.
2. n. A morbid suspension of good sense or judgment, as through fanaticism.

WordNet 3.0

1. n. obsolete terms for legal insanity
2. n. foolish or senseless behavior