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A suffix of Latin origin, occurring in adjectives having originally a passive signification, which is retained more or less fully in adjectives accompanied by verbs derived from the infinitive or perfect participle (English -ate or -it) of the same Latin verb, as in commendable, admirable, dissoluble, etc., habitable, imitable, tolerable, navigable, etc., credible, etc., but is not obvious in adjectives not accompanied by such verbs, as in equable, delectable, horrible, terrible, ignoble, voluble, feeble, etc. In English it is felt and used as a suffix only with the preceding vowel, -able or -ible. See -able, -ible.
(noun)
The Century Dictionary (Public Domain)