1. 
[syn: faithless, traitorous, unfaithful, treasonable, treasonous]
The Collaborative International Dictionary of English v.0.48:
Treasonous \Trea"son*ous\, a.
   Treasonable. --Shak.
   [1913 Webster]
         The treasonous book of the Court of King James.
                                                  --Pepys.
   [1913 Webster]
WordNet (r) 3.0 (2006):
treasonous
    adj 1: having the character of, or characteristic of, a traitor;
           "the faithless Benedict Arnold"; "a lying traitorous
           insurrectionist" [syn: faithless, traitorous,
           unfaithful, treasonable, treasonous]