1650s, "capable of being applied, suitable, appropriate," from Latin stem of apply (v.) + -able. Earlier in this sense was appliable (mid-15c.), and applicable formerly meant "pliable, well-disposed" (1560s).
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]
<a href="https://www.etymonline.com/word/applicability">Etymology of applicability by etymonline</a>
Harper, D. (n.d.). Etymology of applicability. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/applicability