Origin and history of amiability

late 14c., "kindly, friendly," also "worthy of love or admiration," from Old French amiable "pleasant, kind; worthy to be loved" (12c.), from Late Latin amicabilis "friendly," from Latin amicus "friend, loved one," noun use of an adjective, "friendly, loving," from amare "to love" (see Amy).

The form and sense were confused in Old French with amable "lovable" (from Latin amare "to love"), and by 16c. the English word also had a secondary sense of "exciting love or delight," especially by having an agreeable temper and a kind heart. The word was subsequently reborrowed by English in Latin form without the sense contamination as amicable.

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]