A peculiarity of physical or mental constitution or temperament; a characteristic belonging to, and distinguishing, an individual; characteristic susceptibility; idiocrasy; eccentricity.
Baldric: Well my cousin Bert Baldrick, Mr Gainsborough's butler's dogsbody, says that he's heard that all portraits look the same these days 'cause they're painted to a romantic ideal rather than as a true depiction of the idiosyncratic facial qualities of the person in question.
Blackadder: Your cousin Bert obviously has a larger vocabulary than you do, Baldrick.
BBC. Blackadder, season 3: Duel and Duality
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it was an idiosyncrasy of Tolstoy’s to regard such attempts at political reform as fundamentally futile because “how can all that possibly make men happier or better?” Concerned as Tolstoy was, in his own way, with the welfare of mankind, his outlook was intensely individualistic and he profoundly distrusted political movements.—A.M.