Usagi-no-kuso (rabbit dung) is a dialect word from the Shoda Hizen Arita region, and refers to a lower grade of Chinese Gosu.
It contains a large amount of iron, and when combined with Western cobalt, it kills the pure indigo color and gives a subdued taste to the blue indigo color, approximating the color of old-fashioned dyeing. (Shiota Rikizo)
