Webster’s Revised Unabridged Dictionary · 1913
Treasonable
Trea′son·a·ble (·ȧ·b'l), 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.