Source : Webster's Revised Unabridged Dictionary (1913)

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.

         Loyal father of a treacherous son.       --Shak.

         The treacherous smile, a mask for secret hate.
                                                  --Cowper.

   Syn: Faithless; perfidious; traitorous; false; insidious;
        plotting. -- {Treach"er*ous*ly}, adv. --
        {Treach"er*ous*ness}, n.

Source : WordNet®

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