treasonable adj.

c. 1200, "betraying; betrayal of trust; breach of faith," from Anglo-French treson, from Old French traison "treason, treachery" (11c.; Modern French trahison), from Latin traditionem (nominative traditio) "delivery, surrender, a handing down, a giving up," noun of action from past-participle stem of tradere "deliver, hand over," from trans- "over" (see trans-) + dare "to give" (from PIE root *do- "to give"). A doublet of tradition. The Old French form was influenced by the verb trair "betray."

Vpon Thursday it was treason to cry God saue king James king of England, and vppon Friday hye treason not to cry so. [Thomas Dekker, "The Wonderfull Yeare 1603"]

In old English law, high treason (c. 1400) is violation by a subject of his allegiance to his sovereign or to the state (the sense of high here is "grave, serious"); distinguished from petit treason, treason against a subject, such as murder of a master by his servant. Constructive treason was a judicial fiction whereby actions carried out without treasonable intent, but found to have the effect of treason, were punished as though they were treason itself. The protection against this accounts for the careful wording of the definition of treason in the U.S. Constitution.

Trahison des clercs "self-compromised integrity of intellectuals, betrayal or corruption by academics, moralists, journalists, etc., of their vocation," is the title of a 1927 French work by Julien Benda, translated into English in 1928.

In short, intellectuals began to immerse themselves in the unsettlingly practical and material world of political passions: precisely those passions, Benda observed, "owing to which men rise up against other men, the chief of which are racial passions, passions and national passions." ... "Our age is indeed the age of the intellectual organization of political hatreds" he wrote. "It will be one of its chief claims to notice in the moral history of humanity." [Roger Kimball, introduction to 2007 English edition] 

common termination and word-forming element of English adjectives (typically based on verbs) and generally adding a notion of "capable of; allowed; worthy of; requiring; to be ______ed," sometimes "full of, causing," from French -able and directly from Latin -abilis. It is properly -ble, from Latin -bilis (the vowel being generally from the stem ending of the verb being suffixed), and it represents PIE *-tro-, a suffix used to form nouns of instrument, cognate with the second syllables of English rudder and saddle (n.).

A living element in English, used in new formations from either Latin or native words (readable, bearable) and also with nouns (objectionable, peaceable). Sometimes with an active signification (suitable, capable), sometimes of neutral signification (durable, conformable). It has become very elastic in meaning, as in a reliable witness, a playable foul ball, perishable goods. A 17c. writer has cadaverable "mortal."

To take a single example in detail, no-one but a competent philologist can tell whether reasonable comes from the verb or the noun reason, nor whether its original sense was that can be reasoned out, or that can reason, or that can be reasoned with, or that has reason, or that listens to reason, or that is consistent with reason; the ordinary man knows only that it can now mean any of these, & justifiably bases on these & similar facts a generous view of the termination's capabilities; credible meaning for him worthy of credence, why should not reliable & dependable mean worthy of reliance & dependence? [Fowler]

In Latin, -abilis and -ibilis depended on the inflectional vowel of the verb. Hence the variant form -ible in Old French, Spanish, English. In English, -able tends to be used with native (and other non-Latin) words, -ible with words of obvious Latin origin (but there are exceptions). The Latin suffix is not etymologically connected with able, but it long has been popularly associated with it, and this probably has contributed to its vigor as a living suffix.