The Collaborative International Dictionary of English v.0.48:

Lunacy \Lu"na*cy\, n.; pl. Lunacies. [See Lunatic.]
   1. 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. --Brande. --Burrill.
      [1913 Webster]

            Your kindred shuns your house
            As beaten hence by your strange lunacy. --Shak.
      [1913 Webster]

   2. A morbid suspension of good sense or judgment, as through
      fanaticism. --Dr. H. More.

   Syn: Derangement; craziness; mania. See Insanity.
        [1913 Webster]

WordNet (r) 3.0 (2006):

lunacy
    n 1: obsolete terms for legal insanity [syn: lunacy,
         madness, insaneness]
    2: foolish or senseless behavior [syn: folly, foolery,
       tomfoolery, craziness, lunacy, indulgence]