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.
   [1913 Webster]

         Loyal father of a treacherous son.       --Shak.
   [1913 Webster]

         The treacherous smile, a mask for secret hate.
                                                  --Cowper.
   [1913 Webster]

   Syn: Faithless; perfidious; traitorous; false; insidious;
        plotting.
        [1913 Webster] -- Treach"er*ous*ly, adv. --
        Treach"er*ous*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

treacherously
    adv 1: in a disloyal and faithless manner; "he behaved
           treacherously"; "his wife played him false" [syn:
           faithlessly, traitorously, treacherously,
           treasonably, false]