Etymology
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idiosyncrasy (n.)
c. 1600, from French idiosyncrasie, from Latinized form of Greek idiosynkrasia "a peculiar temperament," from idios "one's own" (see idiom) + synkrasis "temperament, mixture of personal characteristics," from syn "together" (see syn-) + krasis "mixture," from PIE root *kere- "to mix, confuse; cook" (see rare (adj.2)).

Originally in English a medical term meaning "physical constitution of an individual;" mental sense "peculiar mixture" of the elements in one person that makes up his character and personality first attested 1660s. In modern use, loosely, one's whims, habits, fads, or tastes. Sometimes confused in spelling with words in -cracy, but it is from krasis not kratos.
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idiosyncratic (n.)
1756, from idiosyncrasy + -ic. Earlier in same sense was idiosyncratical (1640s). Related: Idiosyncratically.
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personality (n.)

late 14c., personalite, "quality or fact of being a person," from Old French personalité and directly from Medieval Latin personalitatem (nominative personalitas), from Late Latin personalis (see personal). Sense of "a distinctive essential character of a self-conscious being" is recorded by 1795, from French personnalité.

Personality is the supreme realization of the innate idiosyncrasy of a living being. It is an act of courage flung in the face of life, the absolute affirmation of all that constitutes the individual, the most successful adaptation to the universal conditions of existence, coupled with the greatest possible freedom of self-determination. [C.G. Jung, "The Development of Personality," 1932]

Meaning "person whose character stands out from that of others" is from 1889. Personality cult "devotion to a leader encouraged on the basis of aspects of his personality, rather than ideological or political considerations," is attested by 1956.

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