1829, "readableness," especially "quality that makes something pleasurable or interesting to read;" from readable + -ity.
Entries linking to readability
readable adj.
early 15c., redable, "legible," from read (v.) + -able. The meaning "of sufficient interest to be read" is by 1771. Related: Readably; readableness.
-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]