The Collaborative International Dictionary of English v.0.48:

Cautionary \Cau"tion*a*ry\, a.
   1. Conveying a caution, or warning to avoid danger; as,
      cautionary signals.
      [1913 Webster]

   2. Given as a pledge or as security.
      [1913 Webster]

            He hated Barnevelt, for his getting the cautionary
            towns out of his hands.               --Bp. Burnet.
      [1913 Webster]

   3. Wary; cautious. [Obs.] --Bacon.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cautionary
    adj 1: warding off; "the swastika...a very ancient prophylactic
           symbol occurring among all peoples"- Victor Schultze
           [syn: cautionary, prophylactic]
    2: serving to warn; "shook a monitory finger at him"; "an
       exemplary jail sentence" [syn: admonitory, cautionary,
       exemplary, monitory, warning(a)]