[syn: punic, perfidious, treacherous]
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):
treacherous
    adj 1: dangerously unstable and unpredictable; "treacherous
           winding roads"; "an unreliable trestle" [syn:
           treacherous, 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, treacherous]
Moby Thesaurus II by Grady Ward, 1.0:
91 Moby Thesaurus words for "treacherous":
   Machiavellian, Punic, ambidextrous, artful, calculating, chancy,
   chiseling, collusive, covinous, crafty, crooked, cunning,
   dangerous, deceitful, deceptive, desultory, dishonest, disloyal,
   double, double-crossing, double-dealing, double-faced,
   double-minded, double-tongued, doublehearted, doubtful, dubious,
   duplicitous, faithless, false, false-principled, falsehearted,
   finagling, fraudulent, furtive, guileful, hairy, hazardous,
   indirect, infirm, insecure, insidious, insubstantial, jeopardous,
   misleading, perfidious, perilous, precarious, precariousness,
   provisional, recreant, risky, rocky, scheming, shaky, sharp,
   shifting, shifty, slippery, sneaky, surreptitious, temporary,
   tentative, ticklish, tottery, traitorous, trickish, tricky,
   two-faced, uncertain, undependable, underhand, underhanded,
   unfaithful, unfaithworthy, unhealthy, unloyal, unpredictable,
   unreliable, unsafe, unsolid, unsound, unstable, unsteadfast,
   unsteady, unsubstantial, unsure, untrue, untrustworthy, wicked,
   wily