late 14c., "beyond expression, too great for words, inexpressible," from Old French ineffable (14c.) or directly from Latin ineffabilis "unutterable," from in- "not, opposite of" (see in- (1)) + effabilis "speakable," from effari "utter," from assimilated form of ex "out" (see ex-) + fari "to say, speak," from PIE root *bha- (2) "to speak, tell, say."
Meaning "that may not be spoken" is from 1590s. Plural noun ineffables was, for a time, a jocular euphemism for "trousers" (1823; see inexpressible). Related: Ineffably.
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]