The Collaborative International Dictionary of English v.0.48:

Betray \Be*tray"\ (b[-e]*tr[=a]"), v. t. [imp. & p. p.
   Betrayed (-tr[=a]d"); p. pr. & vb. n. Betraying.] [OE.
   betraien, bitraien; pref. be- + OF. tra["i]r to betray, F.
   trahir, fr. L. tradere. See Traitor.]
   1. To deliver into the hands of an enemy by treachery or
      fraud, in violation of trust; to give up treacherously or
      faithlessly; as, an officer betrayed the city.
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            Jesus said unto them, The Son of man shall be
            betrayed into the hands of men.       --Matt. xvii.
                                                  22.
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   2. To prove faithless or treacherous to, as to a trust or one
      who trusts; to be false to; to deceive; as, to betray a
      person or a cause.
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            But when I rise, I shall find my legs betraying me.
                                                  --Johnson.
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   3. To violate the confidence of, by disclosing a secret, or
      that which one is bound in honor not to make known.
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            Willing to serve or betray any government for hire.
                                                  --Macaulay.
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   4. To disclose or discover, as something which prudence would
      conceal; to reveal unintentionally.
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            Be swift to hear, but cautious of your tongue, lest
            you betray your ignorance.            --T. Watts.
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   5. To mislead; to expose to inconvenience not foreseen to
      lead into error or sin.
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            Genius . . . often betrays itself into great errors.
                                                  --T. Watts.
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   6. To lead astray, as a maiden; to seduce (as under promise
      of marriage) and then abandon.
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   7. To show or to indicate; -- said of what is not obvious at
      first, or would otherwise be concealed.
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            All the names in the country betray great antiquity.
                                                  --Bryant.
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WordNet (r) 3.0 (2006):

betray
    v 1: reveal unintentionally; "Her smile betrayed her true
         feelings" [syn: betray, bewray]
    2: deliver to an enemy by treachery; "Judas sold Jesus"; "The
       spy betrayed his country" [syn: betray, sell]
    3: disappoint, prove undependable to; abandon, forsake; "His
       sense of smell failed him this time"; "His strength finally
       failed him"; "His children failed him in the crisis" [syn:
       fail, betray]
    4: be sexually unfaithful to one's partner in marriage; "She
       cheats on her husband"; "Might her husband be wandering?"
       [syn: cheat on, cheat, cuckold, betray, wander]
    5: give away information about somebody; "He told on his
       classmate who had cheated on the exam" [syn: denounce,
       tell on, betray, give away, rat, grass, shit,
       shop, snitch, stag]
    6: cause someone to believe an untruth; "The insurance company
       deceived me when they told me they were covering my house"
       [syn: deceive, betray, lead astray] [ant: undeceive]