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. (Etymonline)
Usage Notes •
- When a word ends in –able, the main part of the word (i.e. the bit that comes before the –able ending) is usually a complete word in itself. For example: bearable (from bear), readable (from read), and acceptable (from accept). This is also true when the base word ends in an e that’s dropped before the –able ending is added, e.g. advisable (from advise), inflatable (from inflate), or where it ends in a consonant that’s doubled when the ending is added, e.g. forgettable (from forget), regrettable (from regret).
- If the main part of the word ends with a ‘hard’ c (pronounced like the c in cab) or a ‘hard’ g (pronounced like the g in game) then the ending is always –able, e.g. navigable, amicable.
- Adjectives ending in –able often form opposites with un– (much more likely) or in-. Remember these two words: unfeasible, unintelligible.
- Informal usage often adds –able to verbal phrases, e.g. get-at-able, come-at-able, unputdownable.
Word Formation Resource •