The Collaborative International Dictionary of English v.0.48:

Beware \Be*ware"\ (b[-e]*w[^a]r"), v. t.
   To avoid; to take care of; to have a care for. [Obs.]
   "Priest, beware your beard." --Shak.
   [1913 Webster]

         To wish them beware the son.             --Milton.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Beware \Be*ware"\, v. i. [Be, imperative of verb to be + ware.
   See Ware, Wary.]
   1. To be on one's guard; to be cautious; to take care; --
      commonly followed by of or lest before the thing that is
      to be avoided.
      [1913 Webster]

            Beware of all, but most beware of man ! --Pope.
      [1913 Webster]

            Beware the awful avalanche.           --Longfellow.
      [1913 Webster]

   2. To have a special regard; to heed. [Obs.]
      [1913 Webster]

            Behold, I send an Angel before thee. . . . Beware of
            him, and obey his voice.              --Ex. xxiii.
                                                  20, 21.
      [1913 Webster]

   Note: This word is a compound from be and the Old English
         ware, now wary, which is an adjective. "Be ye war of
         false prophetis." --Wyclif, Matt. vii. 15. It is used
         commonly in the imperative and infinitive modes, and
         with such auxiliaries (shall, should, must, etc.) as go
         with the infinitive.
         [1913 Webster]

WordNet (r) 3.0 (2006):

beware
    v 1: be on one's guard; be cautious or wary about; be alert to;
         "Beware of telephone salesmen" [syn: beware, mind]