1590s, "liable to make answer or defense, accountable," from Anglo-French amenable, from Old French amener "bring, take, conduct, lead" (to the law), from "to" (see ad-) + mener "to lead," from Latin minare "to drive (cattle) with shouts," variant of minari "to threaten," also "to jut, project" (from PIE root *men- (2) "to project"). Sense of "tractable" is from 1803, from notion of "disposed to answer or submit to influence." Related: Amenably.
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]