1892, in C.G. Craddock's translation of Krafft-Ebbing's "Psychopathia Sexualis," a hybrid; see hetero- "other, different" + sexual. The noun is recorded by 1914 but was not in common use until the 1960s. Colloquial shortening hetero is attested from 1933.
-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]