What does it mean for urticaria to be idiopathic?

Unknown. Chronic urticaria (hives) is divided into 2 groups - non-physical & physical. In the latter mast cells release histamine under external stimuli - heat, pressure, cold. 50% of non-physical chronic urticaria is autoimmune, the immune system attacking self. About 5% can be attributed to a chronic infection or malignancy. When your allergist has ruled out these the rest is idiopathic & usually benign.
No cause identified. Idiopathic means that no specific reason for having the hives (urticaria) was found. Most hives that go on longer than a month or so have no cause found and so are idiopathic.