A trait or traits unique or peculiar to a specific individual or group.
A person displaying odd or eccentric behaviors would likely be called idiosyncratic.
Idiosyncratic applies to any individual’s peculiarity, behavior or mannerism. It also includes any peculiar temperamental or physiological expressions. Idiosyncratic also applies to any collective behaviors or language that is particular to any group or individual.
Etymology: It is derived from the term idiosyncrasy, which originates from the Greek idiosynkrasía, which means “a certain habit of the body or a peculiar temperament.”
In a Sentence
An example of idiosyncratic behavior might be, talking to your houseplants to make them grow or if a person only wears purple socks.
Beth liked to wear strange hats and brightly patterned dresses. She was well-known throughout her town for her idiosyncratic appearance.
Albert Einstein was famous for his idiosyncratic ways; it was reported that he never wore socks, he talked to his cat, and he had unusual sleep habits.