The Collaborative International Dictionary of English v.0.48:

Warn \Warn\ (w[add]rn), v. t. [OE. wernen, AS. weornan, wyrnan.
   Cf. Warn to admonish.]
   To refuse. [Written also wern, worn.] [Obs.] --Chaucer.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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. ????.]
   [1913 Webster]
   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.
      [1913 Webster]

            Cornelius the centurion . . . was warned from God by
            an holy angel to send for thee.       --Acts x. 22.
      [1913 Webster]

            Who is it that hath warned us to the walls? --Shak.
      [1913 Webster]

   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.
      [1913 Webster]

   3. To ward off. [Obs.] --Spenser.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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: warn,
       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"

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "warn":
   admonish, advise, alarm, alert, apprehend, apprise, arouse, ask,
   ask for, be imminent, blackmail, bludgeon, bode, bulldoze,
   call for, caution, challenge, charge, claim, clamor for, comminate,
   counsel, croak, cry for, cry havoc, cry out against, cry wolf,
   daunt, demand, denounce, direct, dissuade, encourage, enjoin,
   exact, exhort, expostulate, extort, fly storm warnings, forebode,
   forewarn, frighten, frighten off, give fair warning, give notice,
   give warning, guide, have a premonition, have a presentiment,
   impose, incite, indent, induce, inform, intimidate, issue a caveat,
   issue an ultimatum, kid out of, levy, look black, look threatening,
   lower, make a demand, menace, move, notify, order, order up,
   persuade, place an order, portend, preach, preapprehend, prompt,
   put in requisition, remonstrate, require, requisition, screw,
   sound the alarm, sound the tocsin, startle, talk out of, tell,
   threaten, tip, tip off, unpersuade, urge, utter a caveat,
   utter threats against, warn against