Idiopathic means of unknown origin, occurring spontaneously and not traceable to a direct cause.
When discussing Migraine disease and other headache disorders, probably the most common use we see of the term idiopathic is idiopathic intracranial hypertension. In this condition, The body is either producing too much cerebrospinal fluid or not processing it correctly, and the cerebrospinal fluid pressure becomes too high. The reason this happens is unknown, making it idiopathic.
You can read more about idiopathic intracranial hypertension in Pseudotumor Cerebri (IIH) - The Basics.