\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.