The Collaborative International Dictionary of English v.0.48:

Warner \Warn"er\, n.
   One who warns; an admonisher.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Warner \Warn"er\, n.
   A warrener. [Obs.] --Piers Plowman.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Warner
    n 1: United States filmmaker who with his brothers founded the
         movie studio that produced the first talking picture
         (1881-1958) [syn: Warner, Charles Dudley Warner]
    2: someone who gives a warning to others