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Relating to, constituting, or involving treason: Espionage is a treasonable act.

trea′son·a·bly adv.
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Adv.1.treasonably - in a disloyal and faithless manner; "he behaved treacherously"; "his wife played him false"
References in classic literature ?
When we were all out in the raw air and were steadily moving towards our business, I treasonably whispered to Joe, "I hope, Joe, we shan't find them.
It seems almost treasonably impolite to mention his Thirties' popularity in Royal circles at one stage.
Today, we still see hatred, but it is a nationalistic and populist hatred of outsiders, foreigners, and those who seem "insufficiently or even treasonably nonnationalist" (p.