I have scumbled before. Many times.
Using only your wits, and no online assistance, define in your own words what scumble means, or what scumbling is.
However, not all colors are fully transparent when thinned—in fact, very few actually are. Most colors have a bit of opacity to them, due to the solids used in making the pigments. White, is of course, the most obvious of the non-transparent colors, but others are nearly as opaque as white: Yellow Ochre, Cadmium Red, Cerulean Blue, to name a few.
When an artist thins down a color that is not truly transparent and “glazes” with it, he is said to be scumbling. It has a different visual effect than glazing, because it changes the perception for a different set of reasons. Scumbling actual obscures, to some degree, one’s ability to accurately see the lower layers of color.
Scumbling is, in short, opaque glazing.