Source : Webster's Revised Unabridged Dictionary (1913)

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

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

   Syn: Treacherous; traitorous; perfidious; insidious.
        --{Trea"son*a*ble*ness}, n. -- {Trea"son*a*bly}, adv.

Source : WordNet®

treasonable
     adj : constituting treason; "a traitorous act"; "treasonable acts
           like sabotage" [syn: {traitorous}, {treasonous}]