1. [n]  a reckless impetuous irresponsible person
2. [n]  an insane person
3. [adj]  (informal) some insane and believed to be affected by the phases of the moon

1. \Lu"na*tic\, a. [F. lunatique, L. lunaticus, fr. luna
   the moon. See {Lunar}.]
   1. Affected by lunacy; insane; mad.
   
            Lord, have mercy on my son; for he is lunatic.
                                                  --Wyclif
                                                  (Matt. xvii.
                                                  15).
   
   2. Of or pertaining to, or suitable for, an insane person;
      evincing lunacy; as, lunatic gibberish; a lunatic asylum.
   
   

2. \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.
   
         The lunatic, the lover, and the poet, Are of
         imagination all compact.                 --Shak.
   
   

probably the same as epileptic, the symptoms of which disease were supposed to be more aggravated as the moon increased. In Matt. 4:24 "lunatics" are distinguished from demoniacs. In 17:15 the name "lunatic" is applied to one who is declared to have been possessed. (See DAEMONIAC.)