late 14c., "able to understand, intelligent," from Latin intelligibilis, intellegibilis "that can understand; that can be understood," from intellegere "to understand, come to know" (see intelligence). In Middle English also "to be grasped by the intellect" (rather than the senses). In English, sense of "capable of being understood, that can be understood" first recorded c. 1600. Related: Intelligibly.
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]