The Collaborative International Dictionary of English v.0.48:

Treacherous \Treach"er*ous\, a. [See Treacher.]
   Like a traitor; involving treachery; violating allegiance or
   faith pledged; traitorous to the state or sovereign;
   perfidious in private life; betraying a trust; faithless.
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         Loyal father of a treacherous son.       --Shak.
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         The treacherous smile, a mask for secret hate.
                                                  --Cowper.
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   Syn: Faithless; perfidious; traitorous; false; insidious;
        plotting.
        [1913 Webster] -- Treach"er*ous*ly, adv. --
        Treach"er*ous*ness, n.
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