The Collaborative International Dictionary of English v.0.48:

Treasonable \Trea"son*a*ble\, a.
   Pertaining to treason; consisting of treason; involving the
   crime of treason, or partaking of its guilt.
   [1913 Webster]

         Most men's heads had been intoxicated with imaginations
         of plots and treasonable practices.      --Clarendon.
   [1913 Webster]

   Syn: Treacherous; traitorous; perfidious; insidious.
        [1913 Webster] --Trea"son*a*ble*ness, n. --
        Trea"son*a*bly, adv.
        [1913 Webster]

WordNet (r) 3.0 (2006):

treasonably
    adv 1: in a disloyal and faithless manner; "he behaved
           treacherously"; "his wife played him false" [syn:
           faithlessly, traitorously, treacherously,
           treasonably, false]