The cover of NY’s “Daily News”, July 17, 2018

What is the difference between treason and treachery? And is treasonous a real word?

According to Harry Shaw’s Dictionary of Problem Words and Expressions (1975), “both treachery and treason imply a willful, deliberate betrayal of trust or confidence. Treachery could be applied to the act of being disloyal to a friend or to making unkind statements about someone behind his back. Treason, however, applies solely to betrayal of one’s country, to disloyalty to one’s citizenship, to violation of allegiance to one’s chosen land: “Benedict Arnold committed an act of treason.” All treasonable acts are treacherous, but not all treachery is treason.”

Here are the Oxford English Dictionary‘s definitions for both the nouns and their adjectival forms:

Treachery: “Deceit, cheating, perfidy; violation of faith or betrayal of trust; perfidious conduct.” First used around 1225.

Treason: 1. “The action of betraying; betrayal of the trust undertaken by or reposed in any one; breach of faith, treacherous action, treachery.” First used around 1225. 2. Law.
In old English law treason was either high treason, an offence against the king’s majesty or the safety of the commonwealth, or petit or petty treason, an offence committed against a subject. Petit treason is now punished only as murder, and high treason is usually styled simply treason. Many acts of high treason are now treated as treason felony.

High treason or treason proper: “Violation by a subject of his allegiance to his sovereign or to the state.”

Defined 1350–51 by Act 25 Edw. III, Stat. 5, c. 2, as compassing or imagining the king’s death, or that of his wife or eldest son, violating the wife of the king or of the heir apparent, or the king’s eldest daughter being unmarried, levying war in the king’s dominions, adhering to the king’s enemies in his dominions, or aiding them in or out of the realm, or killing the chancellor or the judges in the execution of their offices. In 1795 the offence was extended to actual or contemplated use of force to make the king change his counsels, or to intimidate either or both of the Houses of Parliament (but from 1848, see also treason-felony n. at sense 4a below). As a result of the Treason Act (1945), the procedure for murder was applied to treason cases.

Treasonous: “Full of or abounding in treason; characterized by treason or treachery; treasonable.” First used in about 1450.

Treacherous: “Of persons, their attributes or actions: Characterized by treachery; deceiving, perfidious, false; disloyal, traitorous.” First used in about 1330.

And here are a few synonyms of treasonous and treacherous:

traitorous (c1380)
treasonful (a1400)
traitorful (c1440)
treasonable (1487)

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