The Collaborative International Dictionary of English v.0.48:

Madman \Mad"man\, n.; pl. Madmen.
   A man who is mad; lunatic; a crazy person.
   [1913 Webster]

         When a man mistakes his thoughts for person and things,
         he is mad. A madman is properly so defined.
                                                  --Coleridge.
   [1913 Webster]

WordNet (r) 3.0 (2006):

madman
    n 1: an insane person [syn: lunatic, madman, maniac]