Relating to, constituting, or involving treason: Espionage is a treasonable act. (adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "treasonable" in a sentence
"Lord Allen may have been wrong in his head, or ill-advised, or foolishly over-zealous, but his ill-tempered upbraiding of the Dublin Corporation for what he called their treasonable extravagance in thus honouring Swift, whom he deemed an enemy of the King, was the act of a fool."
"Government alleged that these persons had been engaged in treasonable conspiracies, and, the President having exercised his legitimate right in suspending the Habeas Corpus Act, he was not prepared to say that the detention of those persons was illegal, or demanded more decided"
"Since then, I have made it my business to establish contacts and relationships in Beijing that would be called treasonable, and for which I should be shot."