peculiarity (n.)

mid-15c., "belonging exclusively to one person," also "special, particular," from Old French peculiaire and directly from Latin peculiaris "of one's own (property)," from peculium "private property," literally "property in cattle" (in ancient times the most important form of property), from pecu "cattle, flock," related to pecus "cattle" (see pecuniary).

The meaning "unusual, uncommon, odd" is by c. 1600 (earlier "distinguished, special, particular, select," 1580s; for sense development, compare idiom). The euphemistic phrase peculiar institution for U.S. slavery is by 1838. Related: Peculiarly.

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]