"capable of being done, accomplished or carried out," mid-15c., from Anglo-French faisible, from Old French faisable "possible, that may be done; easy, convenient," from fais-, stem of faire "do, make," from Latin facere "to make, do, perform" (from PIE root *dhe- "to set, put"). Fowler recommends this word only for those "who feel that the use of an ordinary word for an ordinary notion does not do justice to their vocabulary or sufficiently exhibit their cultivation."
-ity
word-forming element making abstract nouns from adjectives and meaning "condition or quality of being ______," from Middle English -ite, from Old French -ete (Modern French -ité) and directly from Latin -itatem (nominative -itas), suffix denoting state or condition, composed of -i- (from the stem or else a connective) + the common abstract suffix -tas (see -ty (2)).
Roughly, the word in -ity usually means the quality of being what the adjective describes, or concretely an instance of the quality, or collectively all the instances; & the word in -ism means the disposition, or collectively all those who feel it. [Fowler]