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®

treacherous
     adj 1: dangerously unstable and unpredictable; "treacherous winding
            roads"; "an unreliable trestle" [syn: {unreliable}]
     2: tending to betray; especially having a treacherous character
        as attributed to the Carthaginians by the Romans; "Punic
        faith"; "the perfidious Judas"; "the fiercest and most
        treacherous of foes"; "treacherous intrigues" [syn: {punic},
         {perfidious}]