The Collaborative International Dictionary of English v.0.48:

Argumentative \Ar`gu*men"ta*tive\, a.
   1. Consisting of, or characterized by, argument; containing a
      process of reasoning; as, an argumentative discourse.
      [1913 Webster]

   2. Adductive as proof; indicative; as, the adaptation of
      things to their uses is argumentative of infinite wisdom
      in the Creator. [Obs.]
      [1913 Webster]

   3. Given to argument; characterized by argument;
      disputatious; as, an argumentative writer.
      [1913 Webster] --Ar`gu*men"ta*tive*ly, adv. --
      Ar`gu*men"ta*tive*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

argumentative
    adj 1: given to or characterized by argument; "an argumentative
           discourse"; "argumentative to the point of being
           cantankerous"; "an intelligent but argumentative child"
           [ant: unargumentative]