\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.