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.