readable adj.

Middle English reden, ireden, "to counsel, advise," also "to read," from Old English rædan, gerædan (West Saxon), redan, geredan (Anglian) "to advise, counsel, persuade; discuss, deliberate; rule, guide; arrange, equip; forebode; to read (observe and apprehend the meaning of something written), utter aloud (words, letters, etc.); to explain; to learn through reading; to put in order."

This is reconstructed to be from Proto-Germanic *redan, source also of Old Norse raða, Old Frisian reda, Dutch raden, Old High German ratan, German raten "to advise, counsel, interpret, guess," from PIE root *re- "to reason, count."

Cognate words in most modern Germanic languages still mean "counsel, advise" (compare rede). Old English also had a related noun ræd, red "advice," and read is connected to riddle (n.1) via the notion of "interpret." Century Dictionary notes that the past participle should be written red, as it formerly was, and as in lead/led. Middle English past participle variants include eradde, irad, ired, iræd, irudde.  

The sense-transference to "interpret and understand the meaning of written symbols" is said to be unique to English and (perhaps under Old English influence) Old Norse raða. Most languages use a word rooted in the idea of "gather up" as their word for "read" (such as French lire, from Latin legere).

Sense of "make out the character of (a person)" is attested from 1610s. Musical sense of "perform (at first sight) from the notes" is by 1792. To read up "systematically study" is from 1842; read out (v.) "expel by proclamation" (Society of Friends) is from 1788. Read-only in computer jargon is recorded from 1961.

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.