The Collaborative International Dictionary of English v.0.48:

Treachery \Treach"er*y\, n. [OE. trecher["i]e, trichere, OF.
   trecherie, tricherie, F. tricherie trickery, from tricher to
   cheat, to trick, OF. trichier, trechier; probably of Teutonic
   origin. See Trickery, Trick.]
   Violation of allegiance or of faith and confidence;
   treasonable or perfidious conduct; perfidy; treason.
   [1913 Webster]
   [1913 Webster]

         Be ware, ye lords, of their treachery.   --Chaucer.
   [1913 Webster]

         In the council chamber at Edinburgh, he had contracted
         a deep taint of treachery and corruption. --Macaulay.
   [1913 Webster] Treachetour

WordNet (r) 3.0 (2006):

treachery
    n 1: betrayal of a trust [syn: perfidy, perfidiousness,
         treachery]
    2: an act of deliberate betrayal [syn: treachery, betrayal,
       treason, perfidy]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "treachery":
   Machiavellianism, ambidexterity, artifice, bad faith, cunning,
   danger, deceitfulness, desultoriness, dirty pool, dirty trick,
   dirty work, dishonesty, disloyalty, double cross, double-dealing,
   doubleness, doubleness of heart, duplicity, faithlessness,
   falseheartedness, falseness, foul play, hazard, improbity,
   infirmity, insecurity, insolidity, instability, insubstantiality,
   low cunning, perfidiousness, perfidy, peril, perilousness,
   precariousness, risk, riskiness, sellout, shakiness, shiftiness,
   shiftingness, slipperiness, speculativeness, ticklishness,
   treacherousness, treason, two-facedness, unauthenticity,
   unauthoritativeness, undependability, unfaithworthiness,
   unreliability, unsolidity, unsoundness, unsteadfastness,
   unsteadiness, unsubstantiality, unsureness, untrustworthiness,
   wile