Source : Webster's Revised Unabridged Dictionary (1913)

Warn \Warn\ (w[add]rn), v. t. [OE. wernen, AS. weornan, wyrnan.
   Cf. {Warn} to admonish.]
   To refuse. [Written also {wern}, {worn}.] [Obs.] --Chaucer.

Warn \Warn\, v. t. [imp. & p. p. {Warned}; p. pr. & vb. n.
   {Warning}.] [OE. warnen, warnien, AS. warnian, wearnian, to
   take heed, to warn; akin to AS. wearn denial, refusal, OS.
   warning, wernian, to refuse, OHG. warnen, G. warnen to warn,
   OFries. warna, werna, Icel. varna to refuse; and probably to
   E. wary. ????.]
   1. To make ware or aware; to give previous information to; to
      give notice to; to notify; to admonish; hence, to notify
      or summon by authority; as, to warn a town meeting; to
      warn a tenant to quit a house. ``Warned of the ensuing
      fight.'' --Dryden.

            Cornelius the centurion . . . was warned from God by
            an holy angel to send for thee.       --Acts x. 22.

            Who is it that hath warned us to the walls? --Shak.

   2. To give notice to, of approaching or probable danger or
      evil; to caution against anything that may prove
      injurious. ``Juturna warns the Daunian chief of Lausus'
      danger, urging swift relief.'' --Dryden.

   3. To ward off. [Obs.] --Spenser.

Source : WordNet®

warn
     v 1: notify of danger, potential harm, or risk; "The director
          warned him that he might be fired"; "The doctor warned
          me about the dangers of smoking"
     2: admonish or counsel in terms of someone's behavior; "I
        warned him not to go too far"; "I warn you against false
        assumptions"; "She warned him to be quiet" [syn: {discourage},
         {admonish}, {monish}]
     3: ask to go away; "The old man warned the children off his
        property"
     4: notify, usually in advance; "I warned you that I would ask
        some difficult questions"