- -able suffix expressing ability, capacity, fitness, from French, from L. -ibilis, -abilis, forming adjectives from verbs, from PIE *-tro-, a suffix used to form nouns of instrument. In Latin, infinitives in -are took -abilis, others -ibilis; in English, -able is used for native words, -ible for words of obvious Latin origin. The Latin suffix is not etymologically connected with ABLE (Cf. able), but it long has been popularly associated with it, and this has contributed to its survival as a living suffix. It is related to the second syllable of RUDDER (Cf. rudder) and SADDLE (Cf. saddle).
Etymology dictionary. 2014.