exceptionable adj.

late 14c., excepcioun, "act or fact of leaving out or the excluding of" from the scope of some rule or condition, from Anglo-French excepcioun (late 13c. in a legal sense, "formal objection or protest entered by a defendant"), Old French excepcion, from Latin exceptionem (nominative exceptio) "an exception, restriction, limitation; an objection," noun of action from past-participle stem of excipere "to take out" (see except).

From c. 1400 as "a reservation or exemption;" from late 15c. as "something that is excepted."  The exception that proves the rule is from law: exceptio probat regulam in casibus non exceptis, "the exception proves the rule in cases not excepted;" exception here being "action of excepting" someone or something from the rule in question, not the person or thing that is excepted. The figure of speech in to take exception "find fault with, disapprove" is from excipere being used in Roman law as a modern attorney would say objection.

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.