Basically, a disease that just happens for no reason:
Relating to or denoting any disease or condition that arises spontaneously or for which the cause is unknown.
From an article about Natalie Cole:
The family told the Associated Press the singer died of idiopathic pulmonary arterial hypertension, which led to heart failure. [...] Because Cole’s case was idiopathic, the exact cause is unknown.
I think that last sentence is superfluous. It was idiopathic because it was unknown, not the other way around. And even then, you’re just confirming the definition there, really.