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. ????.]
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   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.
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            Cornelius the centurion . . . was warned from God by
            an holy angel to send for thee.       --Acts x. 22.
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            Who is it that hath warned us to the walls? --Shak.
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   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.
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   3. To ward off. [Obs.] --Spenser.
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