The Oxford Thesaurus An A-Z Dictionary of Synonyms - lunacyn.

1) madness, insanity, dementia, craziness, derangement, psychosis, mania: Formerly, it was believed that lunacy fluctuated in accordance with the phases of the moon. 2) folly, foolishness, bad or poor judgement, illogicality, illogic, senselessness, ridiculousness, irrationality, foolhardiness, stupidity: It would be sheer lunacy to confess to something you did not do.