It means "of two colors"; "blotched with white and black". Used extensively in C.S. Lewis' book, "Perelandra", copywrited in 1944.
After Ransom's extended voyage through space in the translucent coffin, he emerged leperous-white on the one side of his body and golden brown on the other from the constant exposure to the sun it received; the green lady laughed and capered with mirth at the sight of his "piebald" stature.
by Woody Woolever September 08, 2003