If you talk about the idiosyncrasies of someone or something, you are referring to their rather unusual habits or characteristics. Everyone has a few little idiosyncrasies = eccentricity, peculiarity
A peculiarity of physical or mental constitution or temperament; a characteristic belonging to, and distinguishing, an individual; characteristic susceptibility; idiocrasy; eccentricity
If you describe someone's actions or characteristics as idiosyncratic, you mean that they are rather unusual. a highly idiosyncratic personality. = eccentric