1540s, of planetary orbits; 1650s, of persons (an instance of eccentricity); 1794, of persons (a quality of eccentricity); from eccentric (adj.) + -ity or from Modern Latin eccentricitatem, from eccentricus. Related: Eccentricities.
1550s, from French eccentrique and directly from Medieval Latin eccentricus (noun and adjective; see eccentric (n.)). Of persons, figurative sense of "odd, whimsical" first recorded 1620s. "Eccentric is applied to acts which are the effects of tastes, prejudices, judgments, etc., not merely different from those of ordinary people, but largely unaccountable and often irregular ..." [Century Dictionary].
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]
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Harper, D. (n.d.). Etymology of eccentricity. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/eccentricity