The Collaborative International Dictionary of English v.0.48:

Lunatic \Lu"na*tic\, a. [F. lunatique, L. lunaticus, fr. luna
   the moon. See Lunar.]
   1. Affected by lunacy; insane; mad; crazy; demented.
      [1913 Webster]

            Lord, have mercy on my son; for he is lunatic.
                                                  --Wyclif
                                                  (Matt. xvii.
                                                  15).
      [1913 Webster]

   2. Of or pertaining to, or suitable for, an insane person;
      evincing lunacy; as, lunatic gibberish; a lunatic asylum.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lunatic \Lu"na*tic\, n.
   A person affected by lunacy; an insane person, esp. one who
   has lucid intervals; a madman; a person of unsound mind.
   [1913 Webster]

         The lunatic, the lover, and the poet,
         Are of imagination all compact.          --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lunatic
    adj 1: insane and believed to be affected by the phases of the
           moon [syn: lunatic, moonstruck]
    n 1: an insane person [syn: lunatic, madman, maniac]
    2: a reckless impetuous irresponsible person [syn: daredevil,
       madcap, hothead, swashbuckler, lunatic, harum-
       scarum]