Idiosyncratic means unique to an individual. Albert Einstein famously had lots of idiosyncratic habits. For example, he rarely wore socks, and he talked to his cat.
One thing that Einstein definitely wasn't was an idiot. Yet idiosyncratic and idiot are related. Idio is ancient Greek for "one's own." An idiosyncratic person is someone who does things in his own way. And the original meaning of idiot was basically "a regular Joe" — an ordinary person who keeps to himself.