1670s, "state of being external," from external + -ity. From 1839 as "that which is external." From 1833 as "undue regard for externals."
Entries linking to externality
external adj.
early 15c., "overt;" by 1590s as "situated or lying outside," from Latin externus "outside, outward" (from exterus; see exterior) + -al (1). This version won out over exterial. Related: Externally.
-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]